Coiled and non-coiled twisted nanofiber yarn torsional and tensile actuators

ABSTRACT

Actuators (artificial muscles) comprising twist-spun nanofiber yarn or twist-inserted polymer fibers generate torsional and/or tensile actuation when powered electrically, photonically, chemically, thermally, by absorption, or by other means. These artificial muscles utilize non-coiled or coiled yarns and can be either neat or comprising a guest. Devices comprising these artificial muscles are also described.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional application of and claims priority andbenefit to U.S. patent application Ser. No. 14/418,811 filed on Jan. 30,2015, entitled “Coiled And Non-Coiled Twisted Polymer Fiber Torsionaland Tensile Actuators” which is the §371 National Phase Application ofPCT/US13/53227, filed on Aug. 1, 2013; and claims priority to U.S.Provisional Application Ser. No. 61/678,340, filed on Aug. 1, 2012,entitled “Coiled And Non-Coiled Nanofibers Yarn Torsional And TensileActuators,” and U.S. Provisional Application Ser. No. 61/784,126, filedon Mar. 14, 2013, entitled “Coiled And Non-Coiled Twisted Nanofiber YarnAnd Polymer Fiber Torsional And Tensile Actuators,” which patentapplications are commonly owned by the owner of the present invention.These patent applications are hereby incorporated by reference in theirentirety for all purposes.

GOVERNMENT INTEREST

This invention was made with government support under Grant Nos.FA9550-09-1-0537 and FA9550-12-1-0211 awarded by the Air Force Office ofScientific Research, Grant AOARD-10-4067 awarded by the Air Force, andMURI Grant No. N00014-08-1-0654 awarded by the Office of Naval Research.The government has certain rights in the invention. This invention alsowas supported by Grant No. AT-0029 from the Robert A. Welch Foundation.

FIELD OF INVENTION

Suitably tethered, twist-spun nanofiber yarns and twisted fibers providehighly reversible electrically, photonically, thermally, or chemicallydriven torsional or tensile actuation without the required presence ofeither a liquid or solid electrolytes. Yarn coiling or fiber coiling,either due to over twist or plying of yarn or fiber, dramaticallyincreases actuator tensile stroke, as does yarn filling with a guestthat changes dimensions during actuation.

BACKGROUND OF INVENTION

Actuator materials and mechanisms that convert electrical, chemical,thermal, or photonic energy to mechanical energy have been sought forover a century. Nevertheless, humankind has had little success inreplicating the wondrous properties of natural muscle, which has meantthat the most advanced prosthetic limbs, exoskeletons, and humanoidrobots lack critically needed capabilities.

Probably no other material has been described for so many fundamentallydifferent types of actuators than carbon nanotubes. Demonstratedelectrically powered and fuel powered nanotube actuators provide up to afew percent actuator stroke and a hundred times higher stress generationthan natural muscle. Large stroke pneumatic nanotube actuators have beendemonstrated that use electrochemical gas generation within nanotubesheets. In other studies, nanotubes have been used either as electrodesor as additives to profoundly modify the response of other actuatingmaterials—like dielectric, ionically conducting, photoresponsive, shapememory, and liquid crystal polymers.

The following provide examples of these diverse types of actuators basedon carbon nanotubes. Electrostatic attraction and repulsion between twonanotubes was used for cantilever-based nano-tweezers [P. Kim, C. M.Lieber, Science 286, 2148-2150 (1999)] and mechanically-based switchesand logic elements [T. Rueckes, K. Kim, E. Joselevich, G Y. Tseng, C.-L.Cheung, C. M. Lieber, Science 289, 94-97 (2000); V. V. Deshpande, H.-Y.Chiu, H. W. Ch. Postma, C. Mikó, L. Forró, M. Bockrath, Nano Letters 6,1092-1095 (2006)]. On the macroscale, electrically powered [R. H.Baughman et al., Science 284, 1340-1344 (1999); U. Vohrer, I. Kolaric,M. H. Hague, S. Roth, U. Detlaff-Weglikowska, Carbon 42, 1159-1162(2004); S. Gupta, M. Hughes, A. H. Windle, J. Robertson, J. Appl. Phys.95, 2038-2042 (2004)] and fuel powered [V. H. Ebron et al., Science 311,1580-1583 (2006)] carbon nanotube actuators provided up to a few percentactuator stroke and a hundred times higher stress generation thannatural muscle. Demonstrated large stroke pneumatic nanotube actuatorsused electrochemical gas generation within nanotube sheets [G. M. Spinkset al., Advanced Materials 14, 1728-1732 (2002)]. Carbon nanotubecomposites with organic polymers provided photoresponsive [S. V. Ahir,E. M. Terentjev, Nature Materials 4, 491-495 (2005)], shape memory [H.Koerner, G Price, N. A. Pearce, M. Alexander, R. A. Vaia, NatureMaterials 3, 115-120 (2004)], and electromechanical [S. Courty, J. Mine,A. R. Tajbakhsh, E. M. Terentjev, Europhysics Letts. 64, 654-660 (2003)]actuators. Previous work has also demonstrated the use of polymer-fillednon-twisted carbon nanotube yarns as thermally powered shape memorymaterials, but reversible actuation was not achieved [P. Miaudet et al.,Science 318, 1294-1296 (2007)]. In other work, dispersed carbonnanotubes or nanotube sheets have been used for electrically heatingthermally actuating materials to provide cantilever deflections [A. T.Sellinger, D. H. Wang, L.-S. Tan, R. A. Vaia, Adv. Mater. 22, 3430(2010); L. Chen, C, Liu, K. Liu, C. Meng, C. Hu, J. Wang, S. Fan, ACSNano 5, 1588 (2011); and Y. Hu, W. Chen, L. H. Lu, J. H. Liu, C. R.Chang, ACS Nano 4, 3498-3502 (2010)]. Major limitations exist for theabove described carbon nanotube artificial muscles, as well as prior artartificial muscles of any type. These limitations include slow response,low stroke or force generation, short cycle life, hysteresis in actuatorresponse, use of electrolytes, or a narrow temperature range foroperation—and in most cases a combination of some of these and otherlimitations (like low energy conversion efficiency).

Artificial muscles based on carbon nanotube artificial aerogel sheetshave been developed that can operate at extreme temperatures (near 0 Kto above 1900 K) where prior-art muscles cannot operate. They providestroke rates and strokes that can exceed 4×10⁴%/s and 250% in onedirection and generate over 30 times higher force than for the sameweight and length natural muscle [A. E. Aliev et al., Science 323,1575-1578 (2009) and A. E. Aliev et al., PCT International Appl. WO2010/019942 A2 (2010)]. Unfortunately, these carbon nanotube musclestypically use thousands of volts of applied potential and cannot bescaled in the thickness direction to provide muscles that can supportheavy loads.

Electrochemically powered multiwalled carbon nanotube (MWNT) yarnmuscles [J. Foroughi et al., Science 334, 494-497 (2011)] can generateover a thousand times larger rotation per length than previous torsionalmuscles based on shape memory alloys [A. C. Keefe, G. P. Carman, SmartMater. Struct. 9, 665-672 (2000)], ferroelectric ceramics [J. Kim, B.Kang, Smart Mater. Struct. 10, 750-757 (2001)] or conducting polymers[Y. Fang, T. J. Pence, X. Tan, IEEE/ASME Trans. Mechatronics 16, 656-664(2011)]. The twist-spun actuating yarn can accelerate a paddle to 590revolutions/minute in 1.2 s [J. Foroughi et al., Science 334, 494-497(2011)] and provide similar torque and mechanical power generation peryarn weight as the gravimetric capabilities of large electric motors.However, these advantages come at a cost. Since actuation arises fromyarn volume changes generated by ion influx during electrochemicaldouble-layer charge injection, overall system gravimetric performance isdegraded by the need for electrolyte, counter electrode, and devicepackaging, which add much more to actuator weight than the actuatingyarn. The liquid electrolyte also limits operating temperature andvoltage, as well as actuation rate and deployment possibilities.

In some invention embodiments, the present invention eliminates the needfor electrolyte, counter electrode, and special packaging by using asolid guest material in the yarn to generate the volume changes thatproduce tensile and torsional actuation. As used herein, the term“tensile actuation” denotes actuation in the length direction of anactuator, regardless of whether the actuator elongates or contracts inthe length direction during an actuation step. In hybrid nanotubemuscles the twist-spun nanotubes confine this actuating guest in bothsolid and molten states, and provide the mechanical strength and helicalgeometry enabling torsional actuation and enhanced tensile actuation.Yarn actuator structure will be engineered to maximize either torsionalor tensile actuation. Reversible actuation will be powered electrically,photonically, or chemically.

Furthermore, with embodiments of the present invention, the Applicanthas provided demonstration of high-cycle-life, large-stroke, andhigh-rate torsional and tensile artificial muscles that:

-   -   (1) Comprise only a neat or hybrid twist-spun nanotube yarn as        the actuating element.    -   (2) Require no electrolyte or counter-electrodes and operate at        low voltages.    -   (3) Can be electrically, chemically, and photonically powered.    -   (4) Deliver over two million reversible torsional actuation        cycles, wherein a hybrid yarn muscle spins a rotor at an average        11,500 revolutions/minute. This rotation rate is 20 times higher        than we previously demonstrated for electrochemical carbon        nanotube muscles and over 20,000 times higher than for previous        muscles based on shape memory alloys, ferroelectric ceramics, or        conducting polymers.    -   (5) Generates a gravimetric torque per muscle weight that is (a)        five times higher than for previous electrochemical torsional        muscles and (b) slightly higher than for large electric motors.    -   (6) Delivers 3% tensile contraction at 1,200 cycles/minute for        over 1.4 million cycles.    -   (7) Delivers 27.9 kW/kg average power density during muscle        contraction, which is 85 times higher than for natural skeletal        muscle. Including times for both actuation and reversal of        actuation, a contractile power density of 4.2 kW/kg was        demonstrated, which is four times the power-to-weight ratio of        common internal combustion engines.    -   (8) Demonstrated a maximum tensile contraction of 10%.    -   (9) While the above demonstrations of (3)-(8) are for hybrid        muscles in which a twist-spun nanotube host confines a volume        expanding guest, the Applicant of the present invention has also        demonstrated torsional and tensile actuation for neat twist-spun        nanotube yarns that are electrothermally heated to incandescent        temperatures. These neat muscles provide 7.3% tensile        contraction while lifting heavy loads at extreme temperatures        where no other high work capacity actuator can survive.    -   (10) Demonstrations include torsional motors, contractile        muscles, and sensors that capture the energy of the sensing        process to mechanically actuate.

Complex coiled fiber geometries are used to dramatically increaseactuator performance compared with that of prior art nanofiber yarnmuscles.

Paraffin waxes are used in some invention embodiments as prototypicalguests in carbon nanotube yarns because of high thermal stability; thetunability of transition widths and temperatures; the large volumechanges associated with phase transitions and thermal expansion; andtheir ability to wet carbon nanotubes. Such waxes have been longinvestigated and commercially deployed as thermally or electro-thermallypowered actuators [E. T. Carlen, C. H. Mastrangelo, Journal ofMicroelectromech. Syst. 11, 165 (2002)]. By confining the actuating waxin the nanosized pores of a carbon nanotube yarn, Applicant has avoidedconventional hydraulic and external heating systems and directly use amuscle-like geometry, where high surface/volume and thermal andelectrical conductivities enhance response rate and a helical geometryenables both torsional rotation and tensile contraction.

In some other invention embodiments, twist insertion and optional fibercoiling is applied to ordinary polymer fibers, like the high strengthpolyethylene and nylon used for fishing line and sewing thread, in orderto obtain high performance artificial muscles that provide torsionalactuation, tensile actuation, or a combination thereof. Like fornanofiber yarn invention embodiments, (1) the need for electrolyte,counter electrode, and special packaging is also eliminated, sinceelectrochemical processes are not required for actuation and (2)reversible actuation can be powered electrically, photonically,thermally, or chemically for the twisted and for the coiled polymerfibers.

Both cost and performance provide major advantages for the twisted andcoiled polymer fibers. While wires of shape memory metals can generategiant stresses and large strokes and provide fast contractions duringelectrothermal actuation, these artificial muscles are veryexpensive—popular high-performance NiTi wires cost about $1400/pound and$1.50/m. In contrast, commercially available polymer fibers that areprecursor to the polymer muscles are inexpensive (typically˜$2.50/pound), and the processes needed to convert the commerciallyfibers to artificial muscles (twist insertion and optional incorporationof conductor) are inexpensive.

Also, competing shape memory metal actuators are heavy and providehysteretic actuation, which makes them difficult to precisely control,since actuation depends upon prior history within a cycle even when theapplied load is constant. Thermally powered shape memory polymer fibersand polymer-filled, non-twisted carbon nanotube fibers can deliver giantstrokes and contractile work capacities [P. Miaudet et al., Science 318,1294-1296 (2007)], but provide largely irreversible actuation.Electrochemically driven fibers of organic conducting polymers alsoprovide large strokes, but have poor cyclability and require anelectrolyte containment system, which adds to system weight and cost.Invention embodiments will eliminate all of these problems.

SUMMARY OF INVENTION

The present invention includes twist-spun nanofiber yarns andtwist-inserted polymer fibers that serve as artificial muscles toproduce torsional and/or tensile actuation.

The present invention further has actuators (artificial muscles)including twist-spun nanofiber yarn and twisted polymer fibers thatgenerate torsional and/or tensile actuation when powered electrically,photonically, thermally, chemically, by absorption, or by other means.These artificial muscles utilize non-coiled or coiled yarns or polymerfibers and can be either neat or including a guest. The presentinvention also includes devices including these artificial muscles.

The foregoing has outlined rather broadly the features and technicaladvantages of the invention in order that the detailed description ofthe invention that follows may be better understood. Additional featuresand advantages of the invention will be described hereinafter that formthe subject of the claims of the invention. It should be appreciated bythose skilled in the art that the conception and the specificembodiments disclosed may be readily utilized as a basis for modifyingor designing other structures for carrying out the same purposes of theinvention. It should also be realized by those skilled in the art thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention as set forth in the appended claims.

It is also to be understood that the invention is not limited in itsapplication to the details of construction and to the arrangements ofthe components set forth in the following description or illustrated inthe drawings. The invention is capable of other embodiments and of beingpracticed and carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein are for the purposeof the description and should not be regarded as limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1A-1D for nanofiber yarns show: tensile load and optional paddlepositions for a two-end-tethered, fully-infiltrated homochiral yarn(FIG. 1A); a two-end-tethered, bottom-half-infiltrated homochiral yarn(FIG. 1B); a one-end-tethered, fully-infiltrated homochiral yarn (FIG.1C); and a two-end-tethered, fully-infiltrated heterochiral yarn (FIG.1D). A homochiral yarn is a yarn having one chirality (which can differbetween yarn twist and coil twist directions) and a heterochiral yarn isone in which different segments have opposite chirality. The depictedyarns are coiled, non-coiled, four-ply, and two-ply, respectively. Thearrows indicate the observed direction of paddle rotation during thermalactuation. Red and green yarn-end attachments are tethers, meaning theyprohibit end rotation—red attachments also prohibit translationaldisplacement.

FIGS. 1E-1G for carbon nanotube yarns are Scanning Electron Microscopy(SEM) micrographs of homochiral coiled yarn fully infiltrated withparaffin wax (FIG. 1E), neat two-ply yarn (FIG. 1F) and neat four-plyyarn (FIG. 1G).

FIGS. 1H-1J are illustrations of the ideal cross-sections for Fermat(FIG. 1H), dual-Archimedean (FIG. 1I), and guest-infiltrated four-plyFermat yarns (FIG. 1J).

FIG. 2A shows (for the configuration of FIG. 1A) a graph of the measureddependence of tensile actuation strain on temperature before (black) andafter (red) paraffin wax infiltration for a two-end-tethered, coiled,homochiral, dual-Archimedean yarn having 130 μm initial diameter, aninserted twist of approximately 4,000 turns/m (per length of theprecursor sheet stack), and an applied stress of 6.8 MPa. Inset:Corresponding actuation data before (black) and after (red) paraffin waxinfiltration for a non-coiled Fermat yarn having 16 μm initial diameter,approximately 20,000 turns/m twist, and an applied stress of 4.8 MPa.Closed and open symbols are for increasing and decreasing temperature,respectively.

FIG. 2B shows (for the configuration of FIG. 1A) a graph ofelectrothermal tensile actuation strain and work capacity duringcontraction in vacuum as a function of applied electrical power for atwo-end-tethered, neat, coiled, homochiral, dual-Archimedean, carbonnanotube yarn having 115 μm diameter and ˜4,000 turns/m of insertedtwist. Insets: Tensile actuation versus estimated temperature for thisyarn (left) and photograph of the incandescent yarn lifting a 10 g load.Closed symbols and open symbols are for increasing and decreasingtemperature, respectively.

FIGS. 3A-3D show (for the configuration of FIG. 1A) graphs ofelectrothermal tensile actuation for two-end-tethered, homochiral,paraffin-wax-filled carbon nanotube yarns. The data in FIG. 3A showtensile actuation strain versus time after 1,400,000 reversible cyclesfor a 11.5 μm diameter, coiled Fermat yarn having approximately 25,000turns/m twist that was driven at 50% duty cycle by a 18.3 V/cm, 20 Hzsquare wave voltage while lifting a load that provided a 14.3 MPastress. FIG. 3B shows tensile actuation for the yarn of FIG. 3A with 109MPa applied tensile stress when driven at 3% duty cycle by 15 ms, 32V/cm square-wave voltage pulses having a period of 500 ms. FIG. 3C showsthe stress dependence of steady-state tensile actuation and contractilework (black and blue data points, respectively) produced by Jouleheating (0.189 V/cm) for a 150 μm diameter, dual-Archimedean yarn havingdifferent levels of inserted twist. FIG. 3D shows tensile strain versustime for the yarn of FIG. 3C with 3,990 turns/m of inserted twist perprecursor sheet stack length, when supporting a 5.5 MPa tensile stressand driven by a 15 V/cm square wave having 50 ms pulse duration and 2.5s period.

FIGS. 4A-4C show graphs of electrothermal torsional actuation fortwo-end-tethered, paraffin-wax-infiltrated, carbon nanotube yarns. FIG.4A shows (for the configuration of FIG. 1B) average revolutions perminute versus cycle number for a 3.9 cm long, half-infiltrated,homochiral Fermat yarn (10 μm diameter and approximately 22,000 turns/mtwist), when excited by a 15 Hz, 40 V/cm, square wave voltage using 50%duty cycle and 41 MPa applied load. Each point on the graph representsthe average speed over 120 cycles. Inset: rotation angle and rotationspeed versus time for one complete cycle. The average rotation speed wasapproximately 11,500 revolutions/minute over nearly 2 million cycles.FIG. 4B shows average revolutions per minute as a function of appliedelectrical power for different tensile loads when using the yarn in FIG.4A, but deploying a heavier paddle. Inset: rotation angle and speedversus time for 51.5 MPa stress. The average speed was 7,600revolutions/minute. FIG. 4C shows (for the configuration of FIG. 1D)static torque versus applied electrical power for a 100 μm diameter, 6.4cm long, fully-infiltrated, heterochiral, dual-Archimedean yarn havingapproximately 3,000 turns/m of inserted twist per stack length. Insets:Greco-Roman catapult configuration used for torque measurements (bottom)and photograph indicating the melting temperature of a paraffin flakeapplied to the surface of the wax-infiltrated yarn (top left).

FIG. 5 shows a graph of tensile actuation as a function of insertedtwist for a neat, homochiral, carbon nanotube, Fermat yarn in theconfiguration of FIG. 1A. The steady-state electrical power applied toobtain yarn contraction was constant (85±2.6 mW/cm) when normalized tothe measured yarn length for each degree of twist, so the input powerper yarn weight was also constant. Mechanical load was constant andcorresponded to 72 MPa stress for the 13.5 μm yarn diameter measured bySEM for the untethered yarn. The lines are guides for the eyes.

FIGS. 6A-6B show graphs of tensile actuation (left axis) and length(right axis) versus applied stress for a homochiral, non-coiled, carbonnanotube, Fermat (FIG. 6A) yarn and a dual-Archimedean carbon nanotubeyarn (FIG. 6B) having 20,000±500 turns/m of inserted twist and about thesame diameter before (17.5±0.5 μm and 16.4±0.9 μm, respectively) andafter paraffin wax infiltration (18.1±0.9 μm and 16.2±1.1 μm,respectively). The configuration of FIG. 1A was used and the electricpower per length was adjusted to be 35±2 mW/cm for each load, whichprovided actuation to far above the temperature (˜83° C.) at whichcomplete melting of the paraffin wax occurred. The lines are guides forthe eyes.

FIGS. 7A-7B are illustrations of horizontal configurations deployed fortorsional actuators. FIG. 7A shows a variant of a two-end-tethered,partially infiltrated yarn motor in which contact with a lateral surfaceprohibits rotation of an attached end weight, but still enables verticalmovement of this mechanical load. FIG. 7 B shows a torsional motor basedon a fully infiltrated yarn that is one-end tethered. In this case theattached mechanical weight can both rotate and translate vertically.

FIG. 8 shows steady-state torsional rotation as a function of electricalpower for a neat, heterochiral, two-ply, Fermat yarn in vacuum (bluetriangles) and the same yarn type in air after wax filling (blackcircles) on increasing and decreasing temperature (filled and opensymbols, respectively). The insets are SEM micrographs showing thestructure of SZ and ZS segments, which were knotted together to make theheterochiral SZ-ZS yarn. The paddle was located between SZ and ZSsegments for the used two-end tethered configuration of FIG. 1D. Thelines are guides for the eyes.

FIGS. 9A-9B show measurement apparatus (FIG. 9A) and a graph of theobserved dependence of paddle rotation angle on yarn immersion depth(FIG. 9B) in acetonitrile and in hexane for an 8 μm diameter,homochiral, Fermat, carbon nanotube yarn having 25,000 turns/m ofinserted twist. Closed and open symbols are for liquid filling andremoval, respectively. The lines are guides for the eyes.

FIG. 10 shows optical images of a non-twisted monofilament nylon fiber(left), this fiber after twist insertion to provide complete coiling(middle), and this coiled fiber after SZ plying (right).

FIGS. 11-12 show scanning electron microscope (SEM) images of amonofilament nylon fiber that has been helically wrapped with aforest-drawn MWNT sheet strip (FIG. 11) and then twist inserted toprovide a highly coiled nylon fiber that is wrapped with the MWNT sheetstrip (FIG. 12).

FIG. 13 shows that thermal mechanical analysis (TMA) during alow-scan-rate temperature indicates little or no hysteresis for a coilednylon fiber muscle in the FIG. 1A configuration, which suggests that theapparent hysteresis at high scan rates is an artifact, due totemperature lag between polymer fiber and thermocouple temperaturesensor.

FIG. 14 shows that there is no significant change in actuator strokeafter more than a million cycles for a coiled nylon fiber muscle that isbeing electro-thermally heated using a MWNT sheet that is wrapped aroundthe muscle. The fiber has the structure shown in FIG. 12 and theactuator configuration of FIG. 1A was used. The graph depicts the strokeversus cycle number when the two-end-tethered coiled yarn was actuatedat 1 Hz when lifting a 21.5 MPa load. The inset shows that there isirreversible polymer creep under this applied load, and that this creepdecreases with increasing cycle number. Each point represents theaverage of 1,000 cycles.

FIGS. 15A-15B provides optical micrographs for a coiled nylon fiberbefore (FIG. 15A) and after (FIG. 15B) dual helical wrapping withforest-drawn MWNT sheet strips.

FIG. 16 shows for the MWNT wrapped coiled nylon fiber of FIG. 15B thetime dependences of applied voltage (a), electrical resistance (b),temperature (c) and generated strain (d) when this coiled nylon fibermuscle loaded is under 21.7 MPa tensile load. The muscle configurationof FIG. 1A was used. The fiber time response is slow because of thelarge fiber diameter, the low power applied during heating, and the slowcooling for this large diameter fiber when special passive or activemeans are not deployed for fiber cooling.

FIG. 17 shows actuation as a function of time for a coiled,dual-Archimedean, carbon nanotube hybrid yarn muscle, where the yarnguest is silicone rubber. Electrothermal actuation was obtained by usingperiodic electrical square-wave pulse heating and the muscleconfiguration of FIG. 1A was used.

FIG. 18A shows a schematic illustration of an apparatus for fibercoiling about a mandrel and the main process variables: the mandreldiameter (D), the turns/length applied to the fiber (r), thetwist/length of coiling applied on the mandrel (R), the force applied tothe fiber (F) and the angle between the fiber and axis of the mandrel(θ).

FIG. 18B shows an optical micrograph of an 860 μm diameter nylon 6monofilament that has been coiled by wrapping on a 0.4 mm diametermandrel.

FIG. 18C shows an optical micrograph of an 860 μm diameter nylon 6monofilament that has been coiled by wrapping on a 2.7 mm diametermandrel.

FIGS. 19A-19B are photographs showing a mandrel-wrapped, coiled nylon 6monofilament fiber (having a positive thermal expansion) when incontracted state at room temperature while supporting a 50 g compressiveload (FIG. 19A) and after heating using hot air (FIG. 19B) to lift thiscompressive load by providing a reversible expansion of coil length. Thecoiled structure was made by wrapping an 860 μm nylon 6 monofilamentabout a 2.7 mm diameter mandrel.

FIGS. 20A-20B are thermomechanical analysis (TMA) graphs of theactuation performance of non-twisted (FIG. 20A) and coiled (FIG. 20B)nylon monofilament. FIGS. 20A-20B depict, respectively, the actuationstrain observed when the actuators are cyclically heated and cooledbetween 20° C. and 180° C. The insets of FIGS. 20A-20B show thecorresponding tensile moduli under these conditions.

FIGS. 21A-21D are photographs of various woven structures made usingcoiled SZ nylon fiber muscles, which were fabricated by twisting nylonfiber to produce complete coiling and then SZ plying the coiled nylonfiber to produce a torque balanced two-ply structure. FIG. 21A picturesa flat braid textile made from four SZ polymer fiber muscles that werederived from non-twisted nylon 6 monofilament. FIG. 21B shows roundbraided ropes made by braiding eight non-coated, SZ nylon 6 yarnmuscles. FIG. 21C shows a round braided structure made from MWNT-coatedSZ nylon 6 yarn muscles. FIG. 21D shows a plain-weave woven structureconstructed by converting a commercially available silver-coated nylon6,6 multi-filament fiber into a SZ nylon muscle. Eight such coiled SZmuscle fibers were incorporated in the warp direction of the plain weavetextile, while cotton yarns were in the weft direction.

FIGS. 22A-21B are pictures of the non-actuated state (“off” state inFIG. 22A) and the electrothermally actuated state (“on” state in FIG.22B) for a McKibben braid that is woven from polyethylene terephthalatepolymer fibers. Electrothermal contractile actuation of the roundedbraid nylon muscle of FIG. 21C (which consists of eight braidedMWNT-coated SZ nylon muscles), which is located at the center of theMcKibben braid, contracts the McKibben braid and opens its porosity. Thepore area increase as a result of electrothermal actuation is 16%.

FIGS. 22C-22D show a shutter in the closed and opened positions,respectively, which were obtained by using one coiled, two-end-tethered,two-ply, SZ twisted, silver-coated nylon fiber muscle for noiseless,reversible electrothermal actuation. The shutter opens from the closedposition (15° slat inclination with respect to the vertical direction inFIG. 22C) to 90° (the fully open position in FIG. 22D) during fiberactuation.

FIG. 23A-23-B shows the use of mandrel coiled nylon 6 monofilamentfibers having a positive thermal expansion for the reversible openingand closing of shutters using changes in ambient temperature. FIG. 23Ashows the shutter in closed position at room temperature and FIG. 23Bshows the shutter in an opened, expanded state at a environmentaltemperature of approximately 80° C. (FIG. 23B). Intermediate temperatureexposures provide intermediate degrees of shutter closing.

DETAILED DESCRIPTION

The present invention is directed to mechanical actuators comprisingtwist-spun nanofiber yarns or twist-inserted polymer fibers as the soleor the predominant actuation material. Unlike most conventionalmechanical actuators, the property changes of an actuating materialcause actuation. For this reason, and because of other similarities inoperation, we call these mechanical actuators artificial muscles.

While artificial muscles have been of practical interest for over 60years, few types have been commercially exploited. Typical problemsinclude slow response, low strain/force generation, short cycle life,use of electrolytes, and low energy efficiency. We have designedguest-filled, twist-spun carbon nanotube yarns having useful topologicalcomplexity as electrolyte-free muscles that provide fast, high-force,large-stroke torsional and tensile actuation. Over a million reversibletorsional and tensile actuation cycles are demonstrated, wherein amuscle spins a rotor at an average 11,500 revolutions/minute or delivers3% tensile contraction at 1,200 cycles/minute. This rotation rate is 20times higher than previously demonstrated for an artificial muscle andthe 27.9 kW/kg power density during muscle contraction is 85 timeshigher than for natural skeletal muscle. Actuation of hybrid yarns byelectrically, chemically, and photonically powered dimensional changesof yarn guest generates torsional rotation and contraction of thehelical yarn host. Demonstrations include torsional motors, contractilemuscles, and sensors that capture the energy of the sensing process tomechanically actuate.

The artificial muscles of invention embodiments comprise a twist-spunnanofiber yarn. For the purpose of invention embodiments, “nanofibers”are defined as fibers that have smallest lateral of below 1000 nm.Networks of electrically interconnected nanofibers having predominatelysmallest nanofiber lateral dimensions of either below 100 nm or below 10nm can be especially useful for different invention embodiments.Nanoribbons are considered to be a specific type of nanofibers.

While the making and/or using of various embodiments of the presentinvention are discussed below, it should be appreciated that the presentinvention provides many applicable inventive concepts that may beembodied in a variety of specific contexts. The specific embodimentsdiscussed herein are merely illustrative of specific ways to make and/oruse the invention and are not intended to delimit the scope of theinvention.

The Fabrication and Structure of Twist-Spun Nanofiber Yarns

The actuator material optimally comprises a network of twist-spunnanofibers that is in the form of a yarn or a material comprising atwist-spun nanofiber yarn, such as a woven textile or a braided or pliedtwist-spun yarn. Various nanofibers and nanofiber syntheses andfabrication processes can be usefully deployed, as can be mixtures ofdifferent nanofiber types and mixtures of nanofibers with othermaterials. As one important example, especially for hybrid actuatingyarns, oriented nanofibers produced by electrostatic spinning can betwist-spun into yarns either during or after electrostatic spinning. Asanother important example, the nanotubes in forest drawn carbon nanotubesheets can be coated with another material as a template (such as aceramic or metal), and then twist spun to make an actuating yarn (whichcan usefully be infiltrated with a guest to make a hybrid actuatingyarn) [M. D. Lima et al., Science 331, 51-55 (2011)]. Depending upon theintended muscle deployment, the nanotube template for this process canoptionally be removed either before or after twist spinning.

Because of their strength, electrical conductivity, and mechanicalstrength, carbon nanotubes (CNTs) are especially preferred for inventionembodiments. Especially useful types of CNTs include carbon multiwallednanotubes (MWNTs), carbon few-walled nanotubes (FWNTs), and carbonsingle-walled nanotubes (SWNTs). Such SWNTs and FWNTs are useful forinvention embodiments even when the nanotube diameter is sufficientlylarge that the SWNTs or FWNTs collapse into ribbons.

Twist-spun nanofiber yarns that comprise nanoribbons of graphene sheetsare especially useful for actuating for embodiments of the invention.One preferred method for making these graphene ribbons as high aspectratio nanofibers is by unzipping carbon nanotubes [D. M. Kosynkin etal., Nature 458, 872-876 (2009)]. This unzipping process can beaccomplished either before or after a CNT array (such as a CNT sheet) istwist spun into a yarn.

Both solid-state and liquid-state processing methods can be used toproduce twisted nanofiber yarns that are useful for inventionembodiments. Some examples of useful solution spinning methods arepolymer-coagulation-based spinning and solution-based spinning methodsthat do not involve a polymer coagulant [B. Vigolo et al., Science 290,1331 (2000); L. M. Ericson et al., Science 305, 1447 (2004); S. Kumar etal., Macromolecules 35, 9039 (2002); and A. B. Dalton et al., Nature423, 703 (2003)]. To provide twisted nanofiber yarns useful forinvention embodiments, yarn twist must be inserted during or after yarnsolution spinning. Additionally, for solution spinning using coagulants(such as a polymer) that remain in the yarn after solution spinning, itis typically useful to remove these coagulants before using these yarnsto make twist-spun yarns of invention embodiments.

Because of these complications in using solution spinning to maketwisted yarns, as well as CNT length degradation during CNT dispersionfor solution spinning, chemical vapor deposition methods that directlyresult in nanotube assemblies that are suitable for spinning arepreferred. Such spinning methods that do not involve dispersion of CNTsin a liquid are referred to as solid-state spinning, whether or notliquids are deployed during or after processing. The resulting yarns aregenerally stronger and able to accommodate higher twist insertion thanneat yarns derived by solution spinning. The first such solid-statespinning method involved chemical vapor deposition (CVD) synthesis ofnanotubes using a floating catalyst and subsequent yarn draw a twistinsertion into a collected CNT aerogel [Y. Li, I. A. Kinloch, A. H.Windle, Science 304, 276 (2004)]. Later methods of twist-based spinninginvolved twist insertion into a nanotube aerogel sheet that has beendrawn from a nanotube forest that has been synthesized on a substrate byCVD. Such twist insertion can be either during sheet draw from a CNTforest [M. Zhang, K. R. Atkinson, R. H. Baughman, Science 306, 1358-1361(2004)] or after a sheet or sheet stack has been drawn from a CNT forest[M. D. Lima et al., Science 331, 51-55 (2011)].

Unless otherwise indicated, the artificial muscle yarns described in theExamples 1-9 were fabricated by first using the following methods tomake twist-spun non-coiled and coiled CNT yarns. Drawable carbon MWNTforests for producing twist-spun yarns were grown by chemical vapordeposition on silicon wafers coated by iron catalyst using acetylene(C₂H₂) gas as the carbon precursor [M. Zhang, K. R. Atkinson, R. H.Baughman, Science 306, 1358-1361 (2004)]. Transmission and scanningelectron microscope (SEM) images of the approximately 350 μm highforests indicate that the MWNTs have an outer diameter of approximately9 nm, contain about 6 walls, and form large bundles. Thermogravimetricanalysis indicates that the amount of non-combustible material in thedrawn nanotubes is below 1 wt %, which places an upper limit on theamount of residual catalyst.

Small and large diameter yarns were fabricated in which twist insertionresulted in three different scroll geometries: Fermat, Archimedean, anddual-Archimedean [M. D. Lima et al., Science 331, 51-55 (2011)]. Smalldiameter yarns were made by symmetrical twist insertion during sheetdraw from a forest or into a pre-drawn nanotube sheet suspended betweeneither a forest and one rigid end support or two rigid end supports.Because of differences in end constraints, these methods provide Fermatscrolls (FIG. 1H) for the former cases of sheets connected to a forestand dual-Archimedean scrolls (FIG. 1I) for the latter case, where tworigid rod supports are used. The yarn diameter could be convenientlyvaried from about 10 μm to about 30 μm by changing the drawn forestwidth from about 0.5 cm to about 5 cm. Much larger diameterdual-Archimedean yarns were typically fabricated by stacking 20 to 40MWNT sheets (1.0 cm to 2.5 cm wide and 5 to 17 cm long) between rigidrods and inserting twist using an electric motor, while one end of thesheet stack supported a 5 g weight that was tethered to prohibitrotation. Approximately 150 turns were necessary to collapse a 5 cm longsheet stack into a 4.5 cm long yarn having dual-Archimedean structure.Introduction of asymmetric stress during twist insertion can convertthese Fermat and dual-Archimedean yarns to Archimedean yarns [M. D. Limaet al., Science 331, 51-55 (2011)].

Fermat yarns directly spun during sheet draw from a forest were used forimmersion-driven torsional actuation; polydiacetylene hybrid yarnmuscles; two-ply yarn muscles; and non-plied, wax-filled torsionalmuscles. The Fermat yarns of FIG. 2A inset; FIGS. 3A-3B; FIGS. 4A-4B;FIG. 5 and FIGS. 6A-6B; were fabricated by drawing a length of nanotubesheet from a forest, and then inserting twist into one end of the sheetvia a motor and a rigid support, while allowing the other end to freelydraw from the MWNT forest. Unless otherwise noted, inserted twist isnormalized with respect to the final yarn length. For these otherinstances, where in most cases twist was inserted in a sheet stack toform a dual-Archimedean yarn, twist was normalized to the length of thesheet stack.

The amount of inserted twist per final yarn length (T) and the finalyarn diameter (d) are important parameters, which for Fermat yarnsdetermine the bias angle (α) between the nanotube orientation on theyarn surface and the yarn direction. Unless otherwise indicated, both dand α were measured by SEM microscopy on yarns that weretwo-end-tethered under tension to prohibit untwist. For Fermat yarns,the theoretical relationship α=tan⁻¹(πdT) is consistent withobservations, despite the complex nature of the realized yarn structure,which contains stochastic elements due to such processes as sheetpleating during twist insertion [M. D. Lima et al., Science 331, 51-55(2011)]. In contrast, since the number of turns inserted by plying twoArchimedean scrolls into a dual-Archimedean scroll (versus the initialnumber of turns that provide twist in each Archimedean scroll) is aconsequence of yarn energetics, a strictly topological equation topredict α from only d and T does not exist. For the same reason, such asimple topological relationship, involving no added parameters, cannotbe obtained for coiled twist-spun yarns of any type.

According to the direction of twist insertion, yarns are classified as Sor Z yarns (for clockwise and anticlockwise twist insertion,respectively). If all segments in a yarn have the same chirality atcorresponding structural levels, the yarn is called homochiral. Thismeans, for instance, that a SZ two ply yarn (with S twist due to plyingand Z twist within each ply) is homochiral. If the yarn has segmentshaving different chirality at the same structural level, then the yarnis called heterochiral. For the heterochiral yarns presently described,different chirality yarn segments are essentially mirror image of eachother (see inset of FIG. 8).

The presently used term “inserted twist” (which is sometimes calledlinking number) is the sum of internal yarn twist and the twist due tocoiling. As done for other structural terms, “yarn diameter” refers tothe diameter of the component yarn even when it is within a coiled orplied structure, and is thereby differentiated from the “coiled yarndiameter” or the “plied yarn diameter”.

Over-twisting MWNT yarns, as for ordinary textile yarns, rubber bands,and DNA molecules, causes coiling (which is called “writhe”). Applicanthas discovered that such coiling, as well as coiling in plied yarn, canbe used to dramatically amplify tensile stroke and work capabilitiescompared with those for uncoiled yarn. Coiled yarns (FIG. 1E) weretypically fabricated under constant load from non-coiled, twist-spunyarns by inserting additional twist until the yarn contracted to 30-40%of its original length. For a dual-Archimedean yarn made under 4 g loadby twist insertion in a stack of 40 co-oriented, 9 mm wide, 15 cm longsheets, coiling started at approximately 580 turns and the yarn wascompletely coiled after approximately 620 turns. Twist insertion untilcomplete coiling occurred (except in the vicinity of the yarn ends)produced a 60% contraction in yarn length.

Four-ply yarns (FIG. 1G and FIG. 1J) were fabricated by inserting Stwist of plying into four identical, parallel-aligned, S-twisted,single-ply Fermat yarns. Two-ply yarns (FIG. 1F) were fabricated asfollows: A ZS yarn was obtained by inserting about 30% extra S twistinto a 11 μm diameter, Fermat S yarn having an initial twist of 20,000turns per meter. This highly twisted yarn was then folded upon itself,so that part of the S twist was converted to Z twist due to plying. A SZyarn was made analogously.

The term coiled yarn is used herein to generically refer to a yarn thathas at least an approximately helical shape in some yarn portions,whether or not this coiling is a result of simple yarn overtwist (likein FIG. 1E) or such processes as yarn plying (FIG. 1F for two-ply yarnand FIG. 1G for four-ply yarn).

Depending upon application needs, nanofiber sheets used for fabricationof twist-spun nanofiber yarns can be optionally densified before twistinsertion. Also, the nanofiber yarns produced by twist spinning canoptionally be densified after or during twist insertion. A particularlyconvenient method for causing sheet densification is by using surfacetension effects due to the process of liquid infiltration and subsequentliquid evaporation.

Electrospinning of nanofibers, and especially polymer nanofibers,provides a useful alternative route to twist-spun nanofiber yarns thatprovide useful hosts for hybrid yarn muscles. In one inventionembodiment these nanofibers are first electrospun into oriented sheetsof nanofibers using electrospinning methods described in the literature[L. S. Carnell et al., Macromolecules 41, 5345-5349 (2008); D. Li, Y.Xia, Advanced Materials 16, 1151-1170 (2004); P. Katta, M. Alessandro,R. D. Ramsier, G. G. Chase, Nano Letters 4, 2215-2218 (2004)]; S. F.Fennessey, R. J. Farris, Polymer 45, 4217-4225 (2004)]. Like for thecase of carbon nanotube sheets, these nanofiber sheets can be twist spuninto yarns. Guests used for carbon nanotube muscles can be providedwithin the host yarn either by guest deposition on the sheets beforetwist spinning or by incorporation of the guest after twist spinning.

Various known methods of twist insertion can be used for introducingtwist during spinning into yarns. Such methods include, but are notlimited to, ring spinning, mule spinning, cap spinning, open-endspinning, vortex spinning, and false twist spinning technologies [See E.Oxtoby, Spun Yarn Technology, Butterworths, 1987 and C. A. Lawrence,Fundamentals of Spun Yarn Technology, CRC Press, 2002].

Twist-spun yarns that comprise nanofibers are especially useful forselected invention embodiments. One reason is in the giant interfacialenergies that arise on the nanoscale enable the convenient confinementof molten guest in a hybrid yarn muscle. Consider, for example, that themolten wax in an actuated wax-filled yarn undergoes a fractional volumedecrease ΔV_(w)/V_(w) when cooled. If this wax volume change occurredwithout decreasing yarn volume, nanotube-paraffin interfacial energies(γ_(np)) would be replaced by nanotube-air interfacial energies (γ_(na))at an energy cost of (γ_(na)−γ_(np))(ΔV_(w)/V_(w))A_(n), where A_(n) isthe gravimetric surface area of the nanotubes. Using γ_(na)−γ_(np)˜18mJ/m² [R. Zhou et al., Nanotechnology 21, 345701 (2010)], A_(n)˜97 m²/g[P. Pötschke, S. Pegel, M. Claes, D. Bonduel, Macromol. Rapid Commun.28, 244 (2008)], and ΔV_(w)/V_(w)˜0.2, about 0.35 kJ/kg of energy isavailable to compress the nanotube yarn as the volume of the liquid waxdecreases. During subsequent yarn actuation by heating and correspondingwax expansion, this elastic energy in the yarn is progressivelyreleased, thereby maintaining coincidence between molten wax and yarnvolume over the entire actuation cycle—as is observed. This analysiscorrectly predicts that excess wax on the yarn surface, as well as waxevaporation, will decrease tensile stroke.

Both very large and very small diameter twisted nanofiber yarns areuseful for embodiments of this invention. However, it should berecognized that (1) the rate of unassisted cooling in ambient air toreverse thermal actuation generally increases as the surface-to-volumeratio of the yarn decreases with increasing yarn diameter and (2) theload carrying capabilities of nanofiber yarns generally increase withincreasing yarn diameter. Single-ply carbon nanotube yarn diameters of˜4 μm to ˜50 μm can be directly twist spun from ˜400 μm high carbonnanotube forests, and increasing forest height and increasing forestdensity increases the yarn diameter obtainable by spinning a given widthof forest. Sheets from ˜400 μm high carbon nanotube forests can bepre-drawn, stacked, and then twist spun to produce single-ply yarns haveseveral hundred micron diameter. These diameters can be dramaticallyincreased by yarn plying and by guest incorporation prior to twistinsertion. By using specialized techniques, carbon nanotube yarndiameters down to ˜100 nm can be twist spun from carbon nanotube forests[W. Li, C. Jayasinghe, V. Shanov, M. Schulz, Materials 4, 1519-1527(2011)]. Also important for micron scale and smaller scale applicationsof invention embodiments, the self-twisting of two nanowires can producea nanoscale plied yarn structure [X.-Y. Ji, M.-Q. Zhao, F. Wei, X.-Q.Feng, Appl. Phys. Lett. 100, 263104 (2012)].

The investigated tensile and torsional actuators were subjected to atleast 30 initial training cycles in order to stabilize the structure ofthe hybrid yarn, and thereby enable highly reversible operation duringsubsequent evaluation for sometimes over 2 million reversible actuationcycles. For the case of thermally powered muscles, these training cycleswere typically to the maximum temperature where the muscle would bedeployed.

Incorporation of Guest in Nanofiber Yarn Muscles

Methods for incorporating guest actuating material into a host yarninclude, for example, melt and solution infiltration (which can befollowed by in-situ polymerization) and biscrolling, where the guest isdeposited on a MWNT sheet before twist insertion [M. D. Lima et al.,Science 331, 51 (2011)]. Some of the methods used for making hybridcarbon nanotube yarns are described in Examples 1-4. Paraffin waxes arepreferred guests because of high thermal stability; the tunability oftransition widths and temperatures; the large volume changes associatedwith phase transitions and thermal expansion; and their ability to wetcarbon nanotube yarns.

As applied in some invention embodiments, biscrolling methods [M. D.Lima et al., Science 331, 51-55 (2011) and M. D. Lima et al., PCT PatentWO2011005375 (A2)] involve deposition of the guest materials onto (1) acarbon nanotube sheet wedge that results from direct twist-basedspinning from a forest or (2) a self-suspended nanotube sheet or sheetstack obtained by sheet draw from a forest. Various host nanofiber webs(i.e., sheets) can also be usefully deployed as described in Section 1.0above. Deposition of guest materials can be accomplished usingconventional methods that can result in a layered stack of guest andhost. More generally, nanofiber webs that are useful for inventionembodiments can comprise nanofibers other than carbon nanotubes, andthese webs can be produced by a process other that sheet draw, such aselectrostatic spinning [L. S. Carnell et al., Macromolecules 41,5345-5349 (2008); D. Li, Y. Xia, Advanced Materials 16, 1151-1170(2004); P. Katta, M. Alessandro, R. D. Ramsier, G. G. Chase, NanoLetters 4, 2215-2218 (2004); S. F. Fennessey, R. J. Farris, Polymer 45,4217-4225 (2004)].

In some embodiments of the present invention, liquid-free deposition ispreferably used. Electrostatic deposition of guest onto the nanotube web(i.e., sheet or sheet wedge) from a carrier gas using an electrostaticpowder coating gun is fast and controllable—attraction between chargedguest particles and the grounded or oppositely charged target web helpsformation of a uniformly deposited layer of guest particles over thedeposition area.

Other liquid-free biscrolling processes involve deposition of the guestmaterial by electron beam evaporation, sputtering, chemical vapordeposition, plasma-enhanced CVD, dry powder airbrush deposition, ordeposition of gas-dispersed guest nanoparticles immediately after theirformation by reaction of gases.

Liquid state and quasi-liquid-state guest deposition also work, such aselectrophoretic deposition; solution filtration-based deposition usingthe nanotube sheet stack as a filter to capture guest nanoparticles;drop casting, and ink jetprinting.

Ink jet printing of guest is effective even for self-supportedindividual nanotube sheets having such low areal density as 1 μg/cm² andcan be conveniently used to provide patterned depositions of one or moreguest materials—thereby leading to engineered variation in guestcomposition along the yarn length and along the yarn diameter. Suchnon-uniform guest deposition can be used to vary actuation along yarnlength.

In the filtration method, (a) solid-state-fabricated nanotube sheetstrips were placed on top of filter paper; (b) liquid-dispersednanoparticles/nanofibers were deposited on top of the nanotube strips byfiltration; (c) the filter paper substrate was dissolved by a solvent;and (d) twist-based spinning on the bilayer ribbon stack wasaccomplished in a liquid bath [M. D. Lima et al., Science 331, 51-55(2011)]. This method can be practiced for any guest nanomaterials thatcan be liquid dispersed, such as by ultrasonication.

When the uniform coating of guest on one side of a sheet or sheet wedgeis replaced with a coating of guest that was only on a fraction of thesheet surface (such as adjacent to one sheet edge) and twist was appliedasymmetrically so that formation of a single Archimedean scroll wasmacroscopically observed, a core-shell yarn structure results where theguest is only in the corridors of the core or shell (depending in parton the wedge half that was preferentially stressed) [M. D. Lima et al.,Science 331, 51-55 (2011)]. By depositing a second guest on the sheetarea that is not covered by the first guest, the second guest can occupythe yarn sheath, while the first guest occupies the yarn core. Thiscore-sheath biscrolling technology [M. D. Lima et al., Science 331,51-55 (2011)] is deployed in invention embodiments to make fuel-poweredtensile and torsional muscles. A catalyst that enables fuel and oxidantreaction to produce heat is preferably in the yarn sheath and a guestthat changes volume when heated is preferably in the yarn core.

Selection of volume changing guests for thermally, electrothermally, andphotothermally powered twist spun muscles is made depending upon volumechanges due to solid-state phase transitions, solid-melt phasetransitions, and solid-state and liquid-state thermal expansioncoefficients in temperature regions removed from phase transitionregions. Paraffin waxes provide the advantage that both the temperatureand sharpness of thermal-dimensional changes are highly tunable. Alsothese waxes are non-toxic. Other long chain molecules, like polyethyleneglycols and fatty acids can also be usefully deployed. These molecules,and like molecules that are deployable for thermal energy storage [S.Mondal, Applied Thermal Engineering 28, 1536-1550 (2008)] can be used asguests in twist-spun muscles, since high enthalpies of phase transitionare usually associated with large volume changes. Organic rotatorcrystals (of which some of these long chain molecules can be classified)are useful since rotational disorder is introduced by solid-statetransitions that can have large associated volume changes [J. M.Pringle, P. C. Howlett, D. R. MacFarlane, M. Forsyth, Journal ofMaterials Chemistry 20, 2056-2062 (2010); G. Annat, J. Adebahr, I. R.McKinnon, D. R. MacFarlane, M. Forsyth, Solid State Ionics 178,1065-1071 (2007); J. Font, J. Muntasell, E. Cesari, Materials ResearchBulletin 30, 839-844 (1995)]. Because of low volatility, plasticcrystals that are ionic crystals are especially useful. One example foractuation at relatively low temperatures is tetraethylammoniumdicyanamide [J. M. Pringle, P. C. Howlett, D. R. MacFarlane, M. Forsyth,Journal of Materials Chemistry 20, 2056-2062 (2010); G. Annat, J.Adebahr, I. R. McKinnon, D. R. MacFarlane, M. Forsyth, Solid StateIonics 178, 1065-1071 (2007)], which undergoes a sharp 5.7% volumeexpansion at a solid-state phase transition that occurs between 17 and20° C.

In applications where the guest material can be prepared in liquid formand later solidified, it is useful to infiltrate the nanofiber yarn whenit still has a low degree of inserted twist. This is advantageousbecause the low-twist nanofiber yarn is still not fully densified bytwist insertion, so there is relatively large amount of void volumebetween the nanofibers. This large void volume (measured as percent oftotal yarn volume) enables the incorporation of a large volume percentof yarn guest, thereby amplifying actuation. This low-twist-infiltrationmethod can be applied, for example, to nanofiber yarn guest that isimbibed into the twisted nanofiber yarn as a precursor liquid resin, andthen polymerized, or to a polymer or polymer mixture that is infiltratedinto a twisted nanofiber yarn while in the molten state and thensolidified. After the resin cure or polymer solidification, if theguest-filled yarn still retains sufficient flexibility, more twist canbe applied to the thereby obtained hybrid yarn in order to fully coilit. If the guest material is applied after coiling the host nanofiberyarn, much less void space would be available in the yarn and thereforeless guest material can be incorporated. Dual-Archimedean yarnscontaining 95% of silicone rubber were prepared in this way (Example21). Silicone rubber is very suitable as guest material for tensileactuators based on hybrid yarns, since it has a large useful workingtemperature range (−55° C. to 300° C.) and a large linear thermalexpansion (3×10⁻⁴/K) for thermal, electrothermal, or photothermalactuation. Due to the high volume percent of guest material that can beincorporated using this low-twist-infiltration method, very largeactuator strokes can be obtained. Using this low-twist-infiltrationmethod, up to 34% tensile contraction under 5 MPa tensile load wasobtained for electrical pulse heating of a coiled carbon nanotube yarncontaining silicone rubber guest (FIG. 17).

Hybrid nanofiber yarn muscles can be optionally made by the process of(a) inserting less twist than required for coiling, (b) infiltrating amolten polymer or an uncured polymer resin, (c) solidifying the polymeror curing the polymer resin and (d) inserting twist sufficient to causeyarn coiling. In fact, the twist inserted before infiltrating a moltenpolymer or an uncured polymer resin can be all or mostly false twist,like obtained by twisting in one direction and then untwisting in theopposite direction.

The weight percent of silicone rubber thereby achieved for the uncoiledand coiled silicone rubber yarn was ˜95%. For hybrid nanofiber yarnscontaining a guest having a density below 2 g/cm³, the preferred volumeloading with guest is above about 50% and the more preferred volumeloading with guest is above about 85%. Also, in instances where a liquidguest or a liquid guest precursor is infiltrated into the host yarn thepreferred volume loading with guest is above about 50% and the morepreferred volume loading with guest is above about 85%. However, forinstances in which the application need is for enhancement of hybridyarn strength rather than enhancement of hybrid yarn stroke, lowervolume percent loadings of guest can be usefully deployed. The volumeweight can be changed according to application needs by varying thedegree of twist that is inserted into the twisted host nanofiber yarnbefore the yarn guest is infiltrated.

The Effects of Actuating Yarn Configuration and Chirality on Tensile andTorsional Actuation

Based on experiments conducted for this invention and theoreticalanalysis to explain experimental results, we can describe configurationsthat optimize either torsional or tensile actuation for yarns that areidentical except that (1) guest infiltration is along the entire yarn orone-half its length, (2) the yarn is homochiral (one chirality) orheterochiral (with equal length segments having opposite chirality), and(3) all infiltrated yarn segments are exposed to the same actuatingconditions. Using opposite chirality yarn segments (like S and Z), witha rotor at their interconnection (FIG. 1D), maximizes initial torque onthe paddle, since these segments operate additively to provide rotation.The one-end-tethered configuration of FIG. 1C provides twice thetorsional rotation of the FIG. 1D configuration, but one-half theinitial torque, so both configurations provide equal torsional workcapacity. Actuation of one segment in a two-end-tethered homochiral yarn(FIG. 1B) generates smaller rotation than for the heterochiral yarn ofFIG. 1D because of the energetic cost of twisting the unactuated yarn asthe actuating yarn untwists. Like for the FIG. 1C configuration, theFIG. 1D configuration with non-plied yarn does not provide reversibleactuation unless internally constrained by a solid guest, to prevent Stwist from cancelling Z twist in the other yarn segment. These sameconfigurations can be usefully deployed for invention embodiments inwhich torsional or tensile actuation is provided by a polymer fiber inwhich twist has been inserted, especially including twist that resultsin yarn coiling.

More detailed analysis is first provided for torsional actuation.Consider that the actuating homochiral yarn in FIG. 1C generates arotation per yarn length of Δθ (degrees/mm) when heated from the initialtemperature to the final actuation temperature. The rotation at anydistance x from the tethered end to the free yarn end is φ(x)=xΔθ, whichis φ(L)=LΔθ at yarn end, where the paddle is located. Paddle rotation atyarn midpoint for the heterochiral yarn with segment lengths L/2 in FIG.1D will be one-half that for the paddle suspended at the end of thehomochiral yarn of FIG. 1C. However, since both S and Z yarns provideequal torque on the paddle of FIG. 1D, the initial torque thataccelerates paddle rotation in this configuration will be double that ofthe homochiral configuration of FIG. 1C. Nevertheless, since this torquedisappears after LΔθ/2 rotations for the heterochiral yarn of FIGS. 1Dand LΔθ rotations for the homochiral yarn of FIG. 1C, the ability toaccomplish torsional work is the same in both cases.

From a view point of combined torsional stroke and torsional workcapability, the half-infiltrated homochiral yarn structure of FIG. 1Bprovides the poorest performance. One-half of the torsional rotationgenerated by untwisting of the actuating yarn segment must be used toup-twist the non-actuating yarn segment, so the paddle rotation producedby the L/2 actuating length during actuation is only LΔθ/4, and nettorque vanishes when this rotation occurs. Even though thishalf-infiltrated FIG. 1B configuration does not optimize torsionalactuation below the guest melting point, this and similar configurationshaving a torsional return spring (which need not be a nanofiber yarn)are the only configurations for a single ply yarn that can provide bothhighly reversible torsional and tensile actuation when the temperaturefor complete melting of the guest is exceeded.

However, the combination of highly reversible torsional and tensileactuation can be obtained for the FIGS. 1C-1D configurations even whenthe guest fully melts if the yarn is two ply (where S twist in the yarnis accompanied by Z twist due to plying for a ZS yarn and the oppositeis true for ZS yarn). The origin of this reversibility is that when ayarn actuates to provide increased twist due to plying, itsimultaneously decreases twist in each of the plied yarns, therebyproviding the returning force that acts to maintain reversibility.

Since present observations show that single-ply actuating yarn segmentsuntwist as they contract during actuation, the yarn configurations thatmaximizes tensile contraction are not those that maximize torsionalactuation. Consider a two-end tethered actuating yarn of FIG. 1A thatprovides a tensile contraction of ΔL/L. When untethered, as in FIG. 1C,the untwist of a homochiral yarn during actuation provides an elongationthat partially cancels tensile contraction. This undesirable elongationcan be largely avoided by using the two-end tethered, one-halfinfiltrated yarn of FIG. 1B, since expansion during untwist of theactuating yarn segment is compensated by contraction during up-twist ofthe unactuated yarn segment. The heterochiral yarn configuration of FIG.1D has decreased tensile performance for a volume expanding guest,especially when yarn diameter and torsional rotation are large, sinceboth yarn segments untwist during actuation, and thereby provideelongations that partially cancel the desired contraction during thermalactuation.

This undesirable partial cancellation of tensile contraction can besignificant: experiments on Fermat yarn contraction during twistinsertion under constant load show that the ratio of percent tensilestrain to inserted twist (degrees per mm of length) near the end oftwist insertion to produce a non-coiled, carbon nanotube yarn is −0.231d %/mm°, where d is yarn diameter. These measurements, which are foryarn bias angle between 22 and 32° and yarn diameters between 9.6 and 15μm, predict that actuating 10 and 100 μm diameter single-ply, Fermatyarn undergoing 100°/mm torsional rotation (in either the FIG. 1C orFIG. 1D configurations) would provide a degradative tensile expansioncomponent due to untwist of 0.23 and 2.3%, respectively. Like the casefor torsional actuation, for a single-ply, twist-spun yarn only the FIG.1A and FIG. 1B configurations can maintain fully reversible tensileactuation when the guest becomes completely fluid during actuation.

The above assumptions of equal length yarn segments and the location ofrotor (i.e., the paddle) at either yarn midpoint or yarn end were madeto avoid any unnecessary complexity for the above discussion. Dependingupon the application needs, the rotor need not be at yarn center. Infact, multiple rotors can be deployed along the yarn length.Additionally, since unactuated yarn lengths serve in many cases as onlytorsional return springs, these unactuated yarn lengths can be replacedby torsional return springs made of various materials, including yarnsor fibers that do not comprise nanofibers. Finally, for cases wheredifferent yarn segments are actuated these yarn segments need not haveopposite chirality or be even made of the same material. For example,one actuated two-ply yarn segment could have Z yarn twist and S twist ofyarn plying while another actuated two-ply yarn segment can have S yarntwist and S twist of yarn plying. As another example, one actuatedsegment could be an n-ply yarn while another yarn segment could be anm-ply yarn (where n and m are different positive integers). In fact, anactuating yarn can optionally include actuating non-yarn segments, suchas a thermally actuated shape memory polymer or a shape metal memorywire. Similarly, twisted polymer fiber muscles can be optionallycombined with other muscle types.

While the configurations of FIGS. 1A-1D indicate a constant tensile loadduring actuation, in many practical applications the tensile load willvary during actuation. This will be the case, for example, when theattachment of the yarn muscle to the constant mechanical tensile load inthese configurations is replaced by attachment of the yarn muscle to aspring whose opposite end is not free to translate. This spring can beof various types, including a cantilever spring.

Also, while the configurations of FIGS. 1A-1D are designed to enabletorsional displacements of a rotor, tensile displacements, or acombination of these displacements, it should be understood that theyarn muscle can also be usefully deployed to provide torque or togenerate forces without the necessity to provide torsional ortranslational displacements.

Performance of Nanofiber Yarn Muscles

The investigated tensile and torsional actuators were subjected to atleast 30 initial training cycles in order to stabilize the structure ofthe hybrid yarn, and thereby enable highly reversible operation duringsubsequent evaluation for sometimes over 2 million reversible actuationcycles. With the exception of described use of actuating yarns ascatapults (Example 12 and related characterization of maximum generatedtorque) and the results of Example 18 and Example 19, all actuatormeasurements were isotonic, meaning that a constant mechanical force wasapplied to the yarn during actuation. Reported gravimetric work andpower capabilities are normalized with respect to the total weight ofthe actuated yarn. The actuator measurements results in Examples 5-19and Example 21 are for twisted carbon nanotube yarns.

Example 5 showed that yarn coiling greatly enhanced thermal tensilecontraction for all yarns, as did guest infiltration (paraffin wax inthis example). Heating a neat coiled yarn from ambient to incandescenttemperature (˜2,560° C.) under 3.8 MPa tensile stress provided areversible yarn contraction of 7.3% (FIG. 2B), corresponding to 0.16kJ/kg work capability.

Example 6 showed that 1,200 cycles per minute and 3% stroke wasdemonstrated for over 1.4 million cycles (FIG. 3A) using atwo-end-tethered, paraffin-wax-filled, coiled Fermat yarn that lifted17,700 times its own weight during electrothermal actuation. Applyingwell-separated 25 ms pulses yielded 1.58% initial contraction and 0.104kJ/kg of mechanical energy during this contraction at an average poweroutput of 4.2 kW/kg, which is four times the power-to-weight ratio ofcommon internal combustion engines.

Example 7 showed that the performance of the yarn muscle of Example 6 asa tensile actuator can be optimized by increasing the applied voltageand mechanical load, while reducing the pulse duration used forelectrothermal actuation. FIG. 3B of this Example 7 showed a series ofactuations wherein the yarn lifted 175,000 times its mass in 30 ms when32 V/cm is applied for 15 ms. The work during contraction (0.836 kJ/kg)provided a power output of 27.9 kW/kg, which is 85 times the peak outputof mammalian skeletal muscles (0.323 kW/kg) and 30 times the maximummeasured power density of previous carbon nanotube muscles [J. Foroughiet al., Science 334, 494 (2011)].

Example 7 and FIG. 3C showed that there is an optimal amount of coilingthat maximizes either stroke or work during contraction for the waxhybrid yarn. A maximum contraction of 5.6% was observed at 5.7 MPastress for a coiled Fermat yarn having intermediate twist. Adding 6.8%more twist to the coiled yarn increased the stress of maximumcontraction (16.4 MPa for 5.1% strain) and the maximum measuredcontractile work (1.36 kJ/kg at 84 MPa), which is 29 times the workcapacity of natural muscle. Contractions of 10% under 5.5 MPa stresswere realized for a 150 μm diameter, partially coiled, dual-Archimedeanyarn by applying well-separated 50 ms, 15 V/cm pulses (FIG. 3D). Sincethe cross-sectional area of this yarn was 170 times higher than for theyarn of FIG. 3A and FIG. 3B, passive cooling in ambient air was lesseffective: the cooling time increased from about 25 ms to about 2.5 s,resulting in a low contractile power density when both heating andcooling times are considered (0.12 kW/kg).

Example 8 for a neat Fermat yarn in the FIG. 1A configuration showed theimportance of twist and resulting bias angle increase on electrothermalcontraction (FIG. 5).

Example 9 demonstrated very fast, highly reversible torsional actuationfor two million cycles for a 10 μm diameter, two-end-tethered,half-wax-infiltrated homochiral Fermat yarn that rotated a paddle atyarn midpoint (FIG. 1B configuration). The hybrid yarn accelerated a16.5 times heavier paddle to a full-cycle-averaged 11,500 rotations perminute—first in one direction and then in reverse (FIG. 4A). Even thoughactuation temperature was far above the temperature of complete waxmelting, this high cycle life resulted because of the presence of atorsional return spring (the unactuated yarn segment of FIG. 1B). FIG.4B shows the dependence of torsional rotation on input electrical powerand applied tensile load for a similar yarn that rotated a 150 timesheavier paddle for a million highly reversible cycles. Increasing loadincreased rotation speed from an average 5,500 revolutions/minute to amaximum average of 7,900 revolutions/minute. Reversible torsionalactuation (12.6°/mm) was also driven for a half-wax-infiltrated yarn byreplacing electrical heating with heating using a light pulse from a 100W incandescent lamp.

Example 10 characterized the effect of wax infiltration on torsionalactuation for a two-end tethered homochiral yarn, wherein one-half ofthe yarn is actuated and the other half largely functions as a torsionalreturn spring. The configuration for the wax containing yarn was exactlythe same as for FIG. 1B, and that for the non-infiltrated yarn differsonly in that the two yarn segments were equivalent except thatelectrical power was applied to only one-half of the yarn length. Inthese comparative examples the same mechanical load was applied and thevoltage used to achieve actuation was identical. Although some torsionalactuation rotation was observed for the neat yarn (4.9°/mm), thisrotation was low compared to the 71.2°/mm electrothermal torsionalactuation observed when one of the yarn segments was subsequentlyinfiltrated with paraffin wax.

Example 11 demonstrated that use of two-ply heterochiral yarn (insteadof a non-plied heterochiral yarn) enables reversible electrothermaltorsional actuation for the FIG. 1D configuration. A SZ yarn wasobtained by inserting about 30% extra twist into an 11 μm diameter,Fermat Z yarn having an initial twist of 20,000 turns per meter. Thishighly twisted yarn was then folded upon itself, so that part of the Ztwist was converted to S twist due to plying. A ZS yarn was madeanalogously. Then these yarns were knotted together, and a paddle wasattached at the position of the knot. The resulting two-ply SZ-ZS yarnstructure was 20 μm in diameter. Steady-state measurements of torsionalactuation as a function of input electrical power measurements (FIG. 8)showed that reversible torsional rotation results in the FIG. 1Dconfiguration for heterochiral, two-ply Fermat yarn that is either (1)wax-filled and actuated to above the melting point of the wax or (2)neat and actuated to incandescent temperature in vacuum. The appliedstresses for these experiments were 3.2 MPa for the neat yarn and 5.8MPa for the wax-filled yarn. While the maximum torsional actuationachieved here for wax-filled SZ-ZS yarn (68°/mm) is about the same asfor the half-infiltrated homochiral yarn of Example 10 in the FIG. 1Bconfiguration (71.2°/mm), the neat SZ-ZS yarn in vacuum provided 30°/mmtorsional actuation (versus the 4.9°/mm for the half-actuated, neat,homochiral yarn of Example 10 in air). Although low for nanotubetorsional actuators, this 30°/mm of torsional actuation for the neatyarn is 200 times the maximum previously reported for shape memoryalloys, ferroelectric ceramics, or conducting polymers. Torsionalactuation was also investigated for this neat two-ply yarn when drivenin vacuum to incandescent temperatures using 9.7 V/cm voltage pulseswith 1 Hz frequency and 20% duty cycle. A 27°/mm rotation was observedwith an average speed of 510 revolutions per minute. This reversiblebehavior contrasts with the lack of reversibility of actuation ofheterochiral, single-ply yarn in the FIG. 1D configuration when the yarndoes not contain solid guest at all points in the actuation cycle. Inthe latter case, permanent cancellation of the opposite twist in the twoyarn segments occurs during actuation, thereby resulting in permanentelongation and reduction of torsional rotation during cycling.

Example 12 demonstrated that paraffin-wax-infiltrated, coiled carbonnanotube yarn can generate giant specific torque and that this torquecan be used to hurl an object. The measured static specific torqueversus applied electrical power for a 100 μm diameter, 6.4 cm long,fully-infiltrated, heterochiral, dual-Archimedean yarn havingapproximately 3,000 turns/m of inserted twist per stack length is shownin FIG. 4C. A maximum specific torque of 8.42 N·m/kg was generated forthis 100 μm diameter yarn, which is five times higher than demonstratedfor electrochemically driven nanotube yarns [J. Foroughi et al., Science334, 494-497 (2011)] and slightly higher than for large electric motors(up to 6 N m/kg).

Example 13 demonstrated reversible, electrothermally powered torsionalactuation for hybrid yarn containing an alternative volume-expandingguest to paraffin wax. This was demonstrated forCH₃(CH₂)₁₁C≡C—C≡C(CH₂)₈COOH, which was infiltrated into twist-spunFermat yarn (d=9 μm and α=26°) and photopolymerized by 1,4-addition toproduce a polydiacetylene (PDA), as described in Example 2. For thefirst investigated horizontal configuration (FIG. 7A, which is analogousto the FIG. 1B configuration) the two-end-tethered homochiral Fermatyarn supported a constant load (2 MPa, when normalized to thecross-section of the unactuated yarn). When 13 mW/cm input power wasapplied, a reversible paddle rotation of 100°/mm was produced as theactuated yarn untwisted during Joule heating. Highly reversibleactuation was demonstrated for over 5,000 on-off cycles, which were themaximum investigated. Additional Example 13 results for aone-end-tethered configuration indicated that the polydiacetylene insidethe yarn functions as an internal torsional spring to enable torsionalactuation to reverse when yarn volume decreases during cooling. Sincethe corresponding neat yarn does not have a return spring, it did notprovide reversible torsional actuation. Due to a few percent volumeincrease at this blue-red phase transition and a larger volume changefrom melting incompletely polymerized monomer at 63° C., reversibletorsional rotation of 100°/mm was obtained for actuation to below 80° C.for the two-end-tethered, half-infiltrated yarn configuration of FIG.1B. Actuation to higher temperatures was poorly reversible, likelybecause of an irreversible phase transition.

Example 14 demonstrated actuation powered by absorption for thepalladium hybrid carbon nanotube yarn of Example 4. The configuration ofFIG. 1D was deployed for characterization of torsional actuation using0.022 MPa applied tensile stress. Reversible torsional actuation waspowered by the absorption and desorption of hydrogen on a 60 nm thickpalladium layer on nanotube bundles within a dual-Archimedean yarn thatrotated at its free end a thousand times heavier paddle during hydrogenabsorption. Injection of 0.05 atm H₂ into a vacuum chamber containingthe actuator caused 1.5 paddle rotations within ˜6 s, which was fullyreversed on a similar time scale during repeated cycling betweenhydrogen exposure and vacuum. Cantilever-based actuators exploiting thedimensional changes of a 10 μm thick Pd alloy layer have been previouslydemonstrated [M. Mizumoto, T. Ohgai, A. Kagawa, J. of Alloys andCompounds 482, 416-419 (2009)], but the response time was in tens ofminutes. The yarn's 100-fold faster response rate resulted from yarnporosity and the thinness of the Pd coating. Such yarn actuators can beused as intelligent muscles that rapidly close an inlet when a targetedhydrogen pressure is exceeded.

Example 15 demonstrated, using the actuator test configuration of FIG.9A, that liquid absorption and desorption can also drive actuation, asshown in FIG. 9B, where torsional actuation of a two-end-tethered Fermatyarn is shown as a function of immersion length in liquid. Largelyreversible torsional rotation was obtained by varying the immersiondepth of a two-end-tethered homochiral yarn in a wetting liquid. Afteran initial training period, the data in FIG. 9B shows that the paddlerotation angle (φ) is a function of yarn immersion depth, withapproximate slopes of 49.6±3.4 and 35.3±1.7 degree/mm for acetonitrileand hexane, respectively.

Example 16 experimentally demonstrated torsional actuation for atwo-end-tethered, homochiral, non-coiled Fermat yarn that was partiallyinfiltrated with polyethylene glycol (PEG), using the method describedin Example 3. PEG was chosen as guest in the carbon nanotube yarn sinceit expands volume by 10% during melting [L. J. Ravin, T. Higuchi, J. Am.Pharm. Assoc. 46, 732-738 (1957)]. Using the FIG. 1B configuration,actuation to above the melting temperature of the PEG was produced byapplying a 2.4 mA square wave current pulse (3.4 Hz frequency and 25%duty cycle) along the entire yarn length. The corresponding power duringactuation was 16 mW/cm and the tensile stress applied during actuationwas 23 MPa (when normalized to the cross-section of the non-actuatedyarn). Using this pulsed electrical power input, a maximum rotationspeed of 1,040 revolutions per minute and a torsional rotation of 37°/mmwere obtained (during an actuation cycle where the infiltrated yarnsegment first untwists during heating and then retwists during unaidedcooling). No degradation in actuation was observed up to the maximumnumber of observed cycles (100,000 cycles).

As an alternative to electrical heating, Example 17 demonstrated thattorsional and tensile actuation of paraffin-containing carbon nanotubehybrid yarns can be produced by incandescent heating from a 100 Wwhite-light lamp that was manually switched on (1.6 to 2 s) and off (0.3to 0.5 s). Using the FIG. 1B configuration, reversible torsionalactuation of 12.6°/mm was obtained for a two-end tethered homochiralFermat yarn (˜15 μm diameter with ˜20,000 turns/m of inserted twist)that was half-infiltrated with paraffin wax. Reversible tensilecontraction occurred simultaneously with untwist of theparaffin-containing yarn segment during torsional actuation caused byphotonically heating this yarn segment.

Examples 18 and 19 demonstrated the use of lever arms and cantileverarms, respectively, to achieve amplified strokes as high frequencies. Atwo-end-tethered, homochiral, coiled Fermat yarn that was fullyinfiltrated with paraffin wax provided actuation. Using a rigid leverarm and a 73 mm long wax hybrid muscle, displacements of 10.4 mm at 6.7Hz and 3.5 mm at 10 Hz were demonstrated. Using a wire cantilever andthe same wax hybrid muscle, a cantilever displacement of 3.4 mm wasachieved at 75 Hz. Relevant to the use of these hybrid twist-spun yarnsto achieve flight for micro-air vehicles, note that these frequenciesare in the range used for insect flight (typically 5 to 200 Hz).

In addition to enabling high cycle life non-electrochemical torsionaland tensile muscles that provide high rate and high specific work andpower capabilities, invention embodiments improve the performance ofprior-art artificial muscles that operate by double layer chargeinjection [J. Foroughi et al., Science 334, 494-497 (2011)]. Theseimprovements include replacement of the non-coiled yarns used forelectrochemical double-layer charging with yarns that are coiled as aresult of writhe produced by over twist or coiled as a result of yarnplying. This coiling increases the tensile actuator stroke that isobtainable for the electrochemically double-layer charged artificialmuscles. While these previous electrochemical muscles could not bereversibly operated using either one-end-tethered configurations orheterochiral configurations because of permanent untwist duringactuation, present invention embodiments show that reversible actuationcan be obtained in both cases (the configurations of FIG. 1C and FIG.1D). This latter improvement results from the present discovery thatsuitably plied yarns can be deployed to obtain reversible actuation evenwhen a liquid inside the actuating yarn is providing the volume changesthat produce actuation. Such suitably plied yarns (like SZ and ZS yarns)are ones in which yarn uncoiling is associated with increase in twistwithin the yarns that are plied. This balance between twist due tocoiling and twist within the yarns acts like a torsional return springto enable reversibility for one-end-tethered and heterochiralconfigurations.

Fabrication of Twist-Inserted, Non-Coiled and Coiled Polymer FiberMuscles

Though polymer fibers used as precursors for making twist-inserted,non-coiled and coiled polymer fiber muscles can be specially made usingwell known methods, advantage has been found in using commerciallyavailable high-strength fibers (either single filament ormultifilament)—most often those used as fishing line or sewing thread.The reason for this present choice is cost, which is decreased by thefact that these fibers are already extensively used commercially formany applications. Also, though the applicants have converted variouscommercially available fibers to artificial muscles that provide over10% reversible contraction when heated, high molecular weightpolyethylene (PE) and nylon 6 or nylon 6,6 fibers having high mechanicalstrength are especially preferred as muscle precursors because of theirhigh strength, especially high realizable contractions during actuation,and commercial availability at relatively low cost.

Like for the carbon nanotube yarn muscles of invention embodiments,twist is inserted into these high strength polymer fibers. This twistmakes the fibers chiral, which enables them to function as torsionalartificial muscles. Most importantly, in order to maximize actuatortensile stroke, the applicants have inserted such a large amount oftwist (measured in turns per meter of initial fiber length) that some ofthis twist is converted to fiber coiling. This coiling is much morecompact than the coiling used to amplify the stroke of shape memorymetal wires. In some preferred invention embodiments the spring index,the ratio of mean coil diameter (i.e., the average of inner and outercoil radius) to fiber diameter is less than about 1.7, whereas for NiTisprings this ratio exceeds 4. In fact, attempts to twist NiTi shapememory wires to lower diameter ratios resulted in wire failure. Thenon-coiled shape memory metal wires contract when heated to above themartensitic to austenitic phase transition, which is reflected inthermal contraction for coiled shape memory wires, while the studiednon-twisted polymer fibers have either a positive or negativefiber-direction thermal expansion coefficient. Independent of the signof the thermal expansion coefficient for the investigatedhighly-oriented fibers, after twist insertion that results in automaticcoiling in the same direction for both fiber twist and fiber coiling,the coiled fiber contract when heated unless adjacent coils are incontact. In this latter case, the coiled fiber can expand as a result ofa temperature increase during actuation.

Twist is most simply inserted into the polymer fibers by attaching theupper fiber end to the shaft of a rotary electric motor and attachingthe lower fiber end to a weight whose rotation is blocked. Fiber coilinggenerally nucleates at one or more locations along the fiber length andthen propagates throughout. The weight applied is important, and isadjustable over a narrow range for a given fiber—too little weight andthe fiber snarls during twist insertion and too much weight and thefiber breaks during twisting. In this range of applied weights, adjacentfiber coils are in contact. Since such coil contact would interfere withcontraction during actuation, larger weights are applied duringactuation to separate the coils. Alternatively, lower weights can beapplied if the fiber is intentionally partially untwisted to separatethe coils, which usually occurs without decreasing the number of coils.The as-coiled fibers will partially uncoil if relative end rotation isallowed. Though thermally annealing can heat-set the coiled fiber,subsequent application of substantial tensile stress will still causecoiling to disappear if end rotation is permitted. While it is possibleto apply sufficiently high load at low temperatures to atwo-end-tethered fiber that the number of coils decreases bycoil-to-twist conversion, this is less successful at high temperaturesbecause the fiber breaks. As an example, for a 130 μm diameter nylon 6(0.005 size monofilament from Coats and Clark) under constant 30 g load,the inserted twist density before the start of fiber coiling is 2046turns/meter±3%, the total inserted twist before the end of coiling is3286 turns/meter±4%, and a total inserted twist of 3653 turns/meter±3%causes the coiled fiber to break.

FIG. 10 (middle) shows an optical image of a highly coiled nylon 6artificial muscle fiber. In order to stabilize this fiber with respectto uncoiling by providing a torque-balanced structure, two fibers havingthe same twist direction (such as right-handed Z twist) were pliedtogether by using an opposite direction of twist for plying (S twist). Athereby torque stabilized SZ nylon yarn fiber muscle is shown in FIG. 10(right). Once thereby stabilized, the plied, highly-coiled fibers can beeasily woven into a textile or a braid. These fiber muscles havehierarchal structures on fiber, plied fiber, and woven structure levels.Hierarchal structure exploitation extends down to the complex moleculararrangement within the fibers, which can produce a negative thermalexpansion in the fiber direction that is ten times more negative than inthe chain direction of polymer crystallites [C. L. Choy, F. C. Chen, andK. Young, J. of Polym. Sci.; Polym. Phys. Ed. 19, 335-352 (1981)].

Optical micrographs of fibers immediately below the onset for fibercoiling show helical surface features (derived from lines in the fiberdirection in the initial fiber), whose bias angle with respect to thefiber direction isα=tan⁻¹(2πrT),where r is the distance from fiber center and T is the inserted twist inturns per fiber length.

As an alternative processing method for producing polymer fiber muscles,a twisted polymer fiber can be helically wrapped around a capillary tubeor any like cylindrical or non-cylindrical mandrel, and subsequently andoptionally released from this mandrel. It has been found that actuationduring heating for the thereby obtained coiled fibers can be either acontraction or an expansion, depending upon the relative directions ofpolymer fiber twist and polymer fiber coiling about the mandrel. Ifpolymer fiber twist density is above a critical level and fiber twistand fiber coiling are in the same chiral direction (either S twist or Ztwist) heating can provide coil contraction, but if in opposite chiraldirections heating can provide coil expansion.

FIG. 18A schematically illustrates a process for simultaneouslyintroducing polymer fiber twist and fiber coiling around a mandrel andFIGS. 18B-18C show examples of coiled polymer fibers made by mandrelwrapping. The exact coil shape and the degree and direction of twistboth for the coil and fiber can be controlled and the geometry of thecoil can be set by thermal annealing. The main variables of mandrelcoiling are the diameter of the mandrel (D), the turns/length applied tothe fiber (r), the twist/length for the coil applied to the mandrel (R),the force applied to the fiber (F) and the angle between the fiber andthe axis of the mandrel (θ). These process variables allow control ofcoil diameter, the spacing between coil turns, and the bias angle andchirality of the coiled polymer fiber muscle. Large diameter coil willproduce higher stroke, but reduced lift capacity. Examples 27 and 28provide methods for critically changing the structure of the coiledyarn, so that either negative or positive coil thermal expansion isobtained, and the performance of the mandrel coiled muscles when they dowork during either contraction or expansion.

In the above described mandrel coiling process, it is possible toindependently select the twist directions for the fiber and for thecoil. This is not possible when twist is applied under load to anotherwise unconstrained fiber. The fiber twist and coiling can be ineither the same or in opposite directions and that will determine thedirection of the coil displacement upon heating: when both the fiber andthe coil have the same direction of twist they will produce a coil thatcontracts during heating if the inserted yarn twist density is above acritical level. When the coil and the fibers have opposite chiralitiesthe coil will expand during heating. This coil expansion can also beused for tensile actuation, where work is accomplished during the coilexpansion part of the work cycle. Again, the larger the diameter of thecoil, the higher will be the stroke, as described in Example 27.

While an electrically conductive muscle component is not required forpolymer fiber muscles or hybrid nanofiber yarn muscles that are drivenchemically, photonically, electromagnetically (by microwave absorption)or by ambient temperature changes, a conductor is needed to provideelectrothermal actuation unless the polymer fiber or nanofiber yarn isitself an electronic conductor (like an intrinsically electronicallyconducting organic polymer metal or a carbon nanofiber yarn). Thisconductor can be, for example, a conducting coating (like a metal orcarbon coating) on a high-strength polymer fiber; metal wire or wires;electronically conducting nanofibers that are helically wrapped about annon-twisted polymer fiber muscle, a coiled polymer fiber muscle, or atwisted but non-coiled polymer fiber muscle; an electronic conductorthat is external to the muscle fiber (like metal wires woven into anactuating textile); or an electronic conductor that is interior to amuscle array (like interior to actuating polymer fiber braids).Commercially available high strength polymer fibers that include anelectrically conducting coating can be deployed, like presentlyevaluated Shieldex Fiber (which is a silver-coated nylon 6,6). The lowresistance per fiber length enables fast heating during electrothermalactuation while applying very low voltages. Additionally, polymer fibersthat are coated with an electronically conducting carbon or carboncomposite can be usefully deployed.

Electronically conducting pastes and inks can be optionally deployed ascoatings to enable electrothermal actuation, such as those containingsilver powder (optionally comprising silver nanofibers or silver flakes)in a binder or forms of electronically conducting carbons in a binder(including carbon nanotubes and graphene flakes). For the preparation ofelectronically conducting coiled polymer fibers, these electronicconductors can be applied to either the non-twisted fiber, the fiberthat is twisted but not coiled, or the coiled fiber.

The preparation of electrothermally actuated or photo-thermally actuatedpolymer fiber muscles by the wrapping of highly conducting nanofibersabout the non-twisted polymer fiber or about twisted or coiled polymerfibers is a useful invention embodiment. It is especially useful todeploy carbon nanotubes as the wrapped nanofibers. These carbonnanotubes can be optionally drawn as sheet ribbons from a carbonnanotube forest and helically wrapped on the polymer muscle fiber. FIG.11 shows a SEM image of a monofilament nylon fiber that has beenhelically wrapped with a forest-drawn MWNT sheet strip. FIG. 12 shows aSEM image of this MWNT-sheet-coated nylon fiber after twist insertion toprovide a highly coiled nylon fiber that is wrapped with the MWNT sheetstrip. FIG. 15A-15B provides optical micrographs for a coiled nylonfiber before (FIG. 15A) and after (FIG. 15B) helical wrapping withforest-drawn MWNT sheet strips. In this case MWNT sheet wrapping hasbeen made on the coiled yarn in two helical wrapping directions. FIG. 16shows the electrothermal actuation obtained for the MWNT wrapped coilednylon fiber of FIG. 15B. Choice of a high thermal conductivityelectronic conductor (as well as high porosity for this electronicconductor, for enhancing heat transport to the environment) can beusefully deployed to decrease the time needed for the cooling thatreverses actuation.

Although non-twisted polymer fiber muscles generally provide smalleractuator contractions than coiled polymer fiber muscles, thesenon-twisted polymer fiber muscles are useful for electrothermallyactuated muscles if the polymer fiber has a large negative thermalexpansion coefficient (preferably above about −10⁻⁴/° C.) and thenon-twisted polymer fiber is wrapped with an electronically conductingnanofiber sheet ribbon. This nanofiber ribbon sheet is preferably acarbon nanotube ribbon sheet, and this carbon nanotube ribbon sheet ispreferably drawn from a carbon nanotube forest. Independent of theorigin of the carbon nanotube ribbon sheet, the optical absorption ofthe carbon nanotubes can use usefully deployed to enhance photo-thermalactuation.

Especially preferred polymers for both non-twisted and twisted polymerfiber muscles are high strength, mechanically drawn nylon 6; nylon 6,6;polyethylene, especially gel-spun polyethylene; and polyvinylidenefluoride. Because of their high strengths, all of these suitable polymerfibers are often used for fishing lines or sewing threads. These polymerfibers that are suitable for conversion to polymer fiber artificialmuscles include fibers that are monofilament, multifilament (withoptional welding or other method of bonding between filaments), andhollow-core multifilament polymer fibers. The polymer multifilamentfibers can optionally comprise polymer filaments having nanoscalediameters, such as those fabricated by electro-spinning or centrifugalspinning. Also, the yarn multifilaments can optionally be of differentpolymers.

Importantly, in order to generate maximum power output during the workpart of an actuator cycle, a coil with a positive thermal expansionshould preferably be operated to do mechanical work during expansion anda coil with a negative thermal expansion should preferably be operatedto do mechanical work during expansion. The reason is that heating rate(which is controlled by power input) can be much higher than coolingrate (which is controlled by ambient conditions). However, when theapplication mode does not require optimization of power density, workduring either expansion or contraction or their combination can be usedto realize performance goals, such as the change in porosity of atextile as a result of a temperature change.

To maximize heating rate (and therefore power output during the workpart of the actuation cycle), while still avoiding local overheating, itcan be useful to deploy profiled applied voltages, currents, or powerinput during actuation. The strategy here is to maximize heating rate atthe start of actuation, where there is no danger of local overheating,and then reduce it when overheating can pose a problem.

Investigated tensile and torsional actuators for highly reversible, longcycle life operation were subjected to most usually at least 30 initialtraining cycles in order to stabilize the structure of the hybridnanofiber yarn or polymer fiber structure, and thereby enable highlyreversible operation during subsequent use for millions of reversibleactuation cycles. For the case of thermally powered muscles, thesetraining cycles were typically to the maximum temperature where themuscle would be deployed.

Performance of Twist Inserted and Coiled Polymer Fiber Muscles

Thermal Actuation.

Unless otherwise indicated, here and elsewhere, force normalization toobtain stress is with respect to the fiber diameter and the fiberdiameter is that of the initial non-twisted fiber. This was done sincethe coil diameter and fiber diameter in the coiled fiber are difficultto accurately measure and since most of the reported measurements areisobaric (constant applied weight), so fiber stress varies duringactuation.

Thermomechanical analysis measurement results for fiber thermalexpansion before and after coiling are shown in FIGS. 20A and 20B,respectively, for a 305 μm diameter nylon 6 monofilament sewing thread(The Thread Exchange, 12 mil). The lack of superposition of increasingand decreasing temperature scans is not due to hysteresis, but isinstead the result of the large temperature scan rate (10° C./minute)and the corresponding lag between fiber temperature and the temperaturemeasured by the thermocouple of the TMA.

The reversible thermal contraction between 20 and 200° C. for the abovenylon 6 fiber increased from 2% to 30% as a result of fiber coiling,providing a stroke amplification factor of 15. This stroke amplificationfactor (SAF) is the ratio of actuator stroke in percent after twistinsertion to that before twist insertion for the same change in ambientand load conditions. A coiled multifilament nylon 6,6 fiber contracts bya similar amount (23%) when using the wider temperature range (20 to220° C.) enabled by the higher temperature where nylon 6,6 isdimensionally stable, thereby providing a SAF of 10. While the maximumtemperature of useful dimensional stability of the gel-spun polyethylenepolymer fiber is much lower (approximately 130° C.), the higher modulusand strength of these fibers is especially useful as a thermally poweredartificial muscle that lifts heavy loads, and provides increased energyefficiency. The non-coiled and coiled Spectra polyethylene providedthermal contractions between 20 and 130° C. of 1.4% and 19%,respectively, corresponding to a SAF of 14.

For polymer fiber muscles coiled in the free state (i.e., without use ofa mandrel) a SAF of above about 5 is preferred and a SAF of above about10 is more preferred. It is also preferred that polymer fiber musclescoiled in the free state have a negative thermal expansion coefficientin the polymer fiber direction before twist insertion.

While other investigated fibers had either a positive or negativelongitudinal thermal expansion coefficient before coiling, when twist isinserted under constant tensile load, they all provided a negativethermal expansion coefficient in the coil axis direction when coiled aslong as fiber twist and fiber coiling are in the same chiral direction.Large diameter (640 μm) nylon 6 monofilament fishing line (BerkeleyTrilene 30 lb test) had an initial average radial thermal expansion of1.8×10⁻⁴/K (between 25 and 130° C.) when twisted under 16 MPa load,which slightly increased to 2.2×10⁻⁴/K at the onset of fiber coiling(where the twist was 540 turns/m and only 1% of the initial fiber lengthwas coiled). The length contraction factor (LCF), which is ratio of theinitial fiber length to the final length of coiled or non-coiled fiber,was 1.16 for this only slightly coiled polymer fiber.

A key advantage of our thermal-expansion-based muscles over muscles thatuse first order phase changes, like shape memory muscles, is theobtainable substantial absence of hysteresis. This hysteresis can easilyexceed 20° C. for NiTi shape memory wires, which greatly complicatesactuator control. As shown in FIG. 13, although our measurementapparatus introduces artificial hysteresis due to differences in fibertemperature and recorded temperature, reducing scan rate to minimizethis effect indicates that the hysteresis of nylon actuator is small ornon-existent. At a 0.3° C./min scan rate, less than 2° C. hysteresis isobservable. This absence of hysteresis, combined with the far morelinear response to temperature compared with that for shape memorywires, makes these coiled polymer fiber muscles well suited for roboticsand artificial prosthetics applications, where a continuous range ofcontrol is desired.

Electrothermal Tensile Actuation.

The first described results on electrical actuation are for a 76 μmdiameter nylon 6 monofilament sewing thread (The Thread Exchange, Inc.,size 003, 5 Tex) that was helically wrapped with a forest-drawn [M.Zhang, S. Fang, A. A. Zakhidov, S. B. Lee, A. E. Aliev, C. D. Williams,K. R. Atkinson, and R. H. Baughman, Science 309, 1215-1219 (2005)]carbon nanotube sheet strip and then coiled to a LCF of 4.5. A 1 Hzsquare wave potential of volt/cm and 20% duty cycle was applied toobtain periodic actuation. The data in FIG. 14 show that this coiledfiber can contract by 10% for over a million cycles when the fiberstress is 22 MPa. Although the coiled fiber did undergo creep (seefigure inset), this creep was less than 2% over the 1 million cycles,the rate of creep decreased with increased cycling, and there was nonoticeable irreversibility in stroke.

When the applied load on a thermally contractile, highly coiled polymerfiber muscle is reduced to low levels, so that neighboring coils startto make contact during tensile contraction, the stroke reduces inmagnitude and eventually becomes positive, and approximately equal tothe radial direction thermal expansion of the fiber. If the amount oftwist in the coiled fiber is decreased by twist removal, the minimumload needed for tensile contraction during actuation is correspondinglyreduced.

The work done during contraction typically increases up to mechanicalloads where the coiled fiber muscle breaks. The observed gravimetricwork during contraction (Example 30) was 5.5 times that obtainable [D.R. Peterson, J. D. Bronzino, Biomechanics: Principles and Applications(CRC Press, Boca Raton, 2008)] for natural muscle. Even more impressive,the average output power (25.5 kW/kg) was 79 times the peak output ofmammalian skeletal muscles (0.323 kW/kg) [D. R. Peterson, J. D.Bronzino, Biomechanics: Principles and Applications (CRC Press, BocaRaton, 2008)].

Muscle cycle rate for these electro-thermally powered artificial musclesis limited by the time required for muscle cooling. Like for all otherthermally or electrochemically driven artificial muscles, this cycletime increases with increasing diameter of the actuating fiber. Whilethis response time is unimportant when thermal cycling times are long,like when using ambient temperature changes to harvest energy fromslowly varying temperature changes or to provide clothing textiles thatchange porosity to provide wearer comfort, it is important whenmaximizing output power during a full cycle of electrothermally poweredactuation.

By employing various active or passive cooling methods, cycle times canbe dramatically reduced. For instance, a 2-ply, coiled, silver-platedfiber (with an initial non-twisted diameter of 170 μm) produced about10% stroke at 5 Hz under 10.8 MPa stress when immersed in water.Similarly, passive operation in helium or active cooling by forcedconvection can allow high-rate actuation of thick fibers.

Photothermal Tensile Actuation.

Thin coiled fibers provide fast tensile actuation when drivenphotothermally using light. For instance, a coiled 76 μm diameter,monofilament nylon 6 fiber (The Thread Exchange, Inc. 3 mil, having 5000turns/m of post-purchase-inserted twist) contracted by 7% in one secondto lift a 26 MPa load when illuminated by a 250 W incandescent lamplocated 2.5 cm from the fiber. The coils were non-contacting at thisstress and the LCF was 2.5.

Chemically Powered Tensile Actuation.

By wrapping nylon 6 fiber with CNT sheet containing a deposition ofcatalytic platinum-black particles and then introducing fiber coiling, achemically powered muscle was produced. This muscle can be powered bythe heat released by various thermal chemical reactions, like the heatreleased by combining fuel with oxidant or the heat produced by chemicaltransformation of a single fluid component (like for Pt catalyzedconversion of hydrogen peroxide in a 30% H₂O₂ aqueous solution towater).

Electrothermal Torsional Actuation.

For initial measurements, a paddle was attached to the free-end of avertically suspended polymer thread that had been twisted to either justbelow the start of fiber coiling or after completion of fiber coiling.In both cases torsional actuation was observed when powered eitherthermally, electrothermally, photonically, or chemically. However, inthis present absence of a torsional return spring, actuation was poorlyreversible—torsional actuator stroke during heating, which correspondedto fiber untwist for non-coiled fiber and untwist of coiling for thefully coiled fiber, rapidly decreased during cycling.

To provide highly reversible torsional actuation for the fishing linesand sewing threads (and other like polymer fibers) even in the absenceof two-end-tethering, fiber coiling was provided in a torque balancedstructure by plying two Z twisted fibers using S coiling to produce a SZtwo-ply fiber. Highly reversible torsional actuation was obtained sincethe sum of fiber Z twist and the S twist of fiber plying must beconserved—coiling during reversal of actuation acts as a torsionalreturn spring for reversing twist release within the fibers. To maximizeobtainable torque, a fiber comprising equal length SZ and ZS segmentswas deployed, with a rotor attached at their midpoint. The therebygenerated gravimetric torque was in the 2.5 to 6 N·m/kg range that istypical for large commercial electric motors.

Applications of Twisted and Coiled Nanofiber Yarns and Twisted andCoiled Polymer Fibers

Since the realized tensile strokes and work and power densities are sohigh for coiled hybrid yarns, these high-cycle-life muscles can be usedfor diverse applications. The major competing NiTi shape memory metalactuators have a highly hysteretic actuator stroke, so control ofactuator displacement is greatly complicated by the dependence ofactuation on prior history within a cycle. This history dependence issmall for the wax hybrid yarn results of FIG. 2A, and can be negligiblefor cycling a neat yarn or any wax-filled yarn between molten states.Also, this hysteresis can be small for twist-inserted polymer fibers,including those that are coiled.

While shape memory metals have been exploited as torsional actuators,the torsional actuation (0.15°/mm) [A. C. Keefe, G. P. Carman, SmartMater. Struct. 9, 665 (2000)] is much smaller than demonstrated here forwax hybrid yarns (71°/mm in Example 10) and polydiacetylene hybrid yarns(100°/mm in Example 13) or for twist-inserted polymer fibers. Improvedcontrol and large rotational actuation, along with potentially longercycle life, indicate the utility of nanofiber yarn muscles andtwist-inserted polymer fiber muscles in medical devices, robots, andshutters, for which shape memory alloys are currently employed, as wellas extension to microvalves, mixers, smart phone lenses, positioners andtoys. Example 22 demonstrates the use of a single coiled, two-ply, SZtwisted, silver-coated nylon fiber as a noiseless actuator forcontrolling the opening of window shutters and FIG. 22C-22D providephotographs of shutters that have been reversibly actuated betweenclosed and opened positions.

Example 38 and FIGS. 23A-23B show the use of two mandrel-coiled nylon 6monofilament fibers for the opening and closing of shutters in responseto ambient temperature changes. As a consequence of opposite directionsof twist insertion in the nylon yarn and mandrel coiling, this coilednylon fiber muscle has a positive thermal expansion coefficient.Operating to do work under compressive load, these nylon musclesprogressively open the shutter as temperature increases. The shuttersconfigurations of Example 22 and Example 38 can be used to controleither air flow or the passage of light.

The nanofiber yarn muscles and the twist-inserted polymer fiber musclescan also be used as macro- or micro-sized pumps, valve drives, andfluidic mixers. These mixers will be useful for “chemical laboratorieson a chip” where chemical analysis or chemical synthesis is efficientlydone in an extremely small area device. Unlike for previouselectrochemically-powered carbon nanotube yarn micro-fluidic mixers [J.Foroughi et al., Science 334, 494 (2011)], no counter-electrode orelectrolyte is required for the yarn torsional muscles or the polymerfiber muscles of present invention embodiments. On the microscale orsmaller scales, the demonstrated torsional actuation can be used forrotating electrodes used in highly sensitive electrochemical analyteanalysis, thereby eliminating need for much larger and much moreexpensive ordinary motors.

The twist-spun yarn muscles and the twist-inserted polymer fiber musclescan additionally be used to eliminate the bulky electromagnets used toprovide actuation for conventional electronic relays and for suchpurposes as actuating the locks on car doors.

The flexibility of twist spun yarn muscles and twist-inserted polymerfiber muscles can be exploited to amplify the stroke realized for agiven length of nanofiber yarn or polymer fiber assembly (in which amuch longer length of actuating yarn is contained). This amplificationcan be achieved by simply wrapping the actuating yarn (or twistedpolymer fiber) about two parallel stationary pins, anchoring one yarnend, and using the opposite yarn end to transmit tensile actuation.However, to minimize friction, these pins can support independent yarnon fiber size pulleys.

Yarn and fiber flexibility and knottability can also be exploited byusing these tensile-actuating nanofiber yarns and twist-inserted polymerfibers to enable the morphing of tensegrity structures. In this casenanofiber hybrid yarns or twist-inserted polymer fibers can provideend-to-end connectivity between the rods used for the tensegritystructures, so as to enable structural morphing. Alternatively, by usinghollow rods in these structures, and connecting the actuating nanofiberhybrid yarns or twist-inserted fibers at opposite rod ends from wherethe yarns or fibers emerge to connect to another rod, the amplitude ofmorphing for the tensegrity structures can be increased. Either thereversible actuation or the irreversible actuation of polymer fibermuscles or nanofiber yarn muscles can be used to provide actuation forvarious other morphing structures, like those folded into orgami foldedshapes and those based on compliant mechanisms (Handbook of CompliantMechanisms, L. L. Howell, S. P. Magleby, and B. M. Olsen, eds., JohnWiley and Sons, Inc., 2013). The coiled polymer fiber muscles and coilednanofiber yarn muscles can be used for either reversibly or irreversiblyopening or closing structures that are folded to make them morecompact—like folded solar cell arrays that are opened when deployed inspace. This deployment can be accomplished electrothermally or byexploiting changed in ambient temperature or exposure to light.

The tensile actuation of twist-spun nanofiber muscles and twist-insertedpolymer fiber muscles can conveniently be deployed on micro and macroscales for peristaltic pumps by configuring independently actuate-ablenanofiber yarn or twisted polymer fiber segments about collapsibletubing that contains the fluid to be pumped, and sequentially actuatingthese yarn or polymer fiber segments to obtain pumping.

Optical device applications provide other invention embodiments.Examples are twist-spun nanofiber yarn and yarn assemblies andtwist-inserted fibers and fiber assemblies that provide ultra-fastoptical shutters (by torsional rotation of a paddle), translation oflight diffusers for laser speckle reduction, means for the translationor rotation of other optical elements, or means to change the focallength of lenses (such as by mechanical deformation of a compliantlenses). Either tensile or torsional actuation of a nanofiber yarn or atwist-inserted polymer fiber muscle can be used to change the pixels onsigns by either rotating these pixels to reveal a different color or bya translation that achieves the same effect.

The nanofiber yarn muscles and twist-inserted polymer fiber muscles canalso be used in haptic devices, wherein actuator displacement providestactile information. This tactile feedback can be through the user'shands or fingers, such as in gloves worn by a surgeon while remotelyconducting surgery using robotic devices. In another application, theseartificial muscles can be used to depress spring-loaded pins in arefreshable Braille display.

Both torsional and tensile actuation of hybrid twist-spun yarns andtwist inserted fibers can be deployed for actuating smart surfaces onlocal and larger scales. For instance, tensile actuation can reversiblyretract spring-loaded pins (which can optionally be cylindrical, bladeshaped, or of other more complicated shapes), thereby changing surfaceroughness. One or more cycles of pin extension and retraction can beused to remove marine organisms from the surface of marine vehicles.Rotation of micron size or larger paddles can also be utilized forchanging surface roughness. These actuating surfaces with controlledroughness can be used to affect the boundary layers in fluids, therebyenabling more efficient and/or controlled motion, such as for marine,air, and land vehicles. Such actuation, like the torsional rotation of apaddle so that either hydrophilic or hydrophobic paddle surface isexterior, can also be used to change surface energy. Such surface energychanges can also be usefully accomplished by the reversibly extensionand contraction of pins.

Like natural muscle and thermal shape memory metal muscles, thethermally actuated nanofiber muscles and twist-inserted polymer fibermuscles require energy input to maintain muscle contraction andcorresponding tensile force generation. For the thermal muscles thismaintenance energy corresponds to that needed to keep muscle temperatureconstant when the temperature exterior to the muscle is lower than theactuated muscle. A similar problem exists for dielectric muscles andelectrochemical muscles, but in these cases the maintenance energyneeded is to replace electrical energy due to electrical self-discharge.This problem of maintenance energy does not arise when actuation isreversed as soon as the work of contraction is accomplished, and can beavoided by using a latch to maintain the force or extent of contraction.Such latch mechanisms have been described in the literature, includinginch-worm actuator mechanism that can increase actuator stroke by anarbitrarily large amount by combining the actuator strokes of multipleactuator cycles [“Fuel Powered Actuators and Methods of Using Same”, R.H. Baughman, V. H. Ebron, Z. Yang, D. J. Seyer, M. Kozlov, J. Oh, H.Xie, J. Razal, J. P. Ferraris, A. G. MacDiarmid, W. A. Macaulay, U.S.Pat. No. 8,096,119 B2].

While small diameter hybrid yarn muscles provide giant gravimetric powerdensities even when the work of contraction is normalized to the entirecycle time (a demonstrated 4.2 kW/kg, from Example 6, which is fourtimes the power-to-weight ratio of common internal combustion engines),the absolute work and power output is low unless yarn diameter isincreased or many yarns are placed in parallel. However, Example 7 showsthat dramatically increasing yarn diameter decreases cooling rate inambient air. This problem can be ameliorated for both the nanofiber yarnand twist-inserted polymer fiber muscles by using active cooling toreverse actuation or by using surrounding ambient temperature media thatrapidly absorb the heat of actuation. Water is one such useful media forcooling. Other useful choices for passive cooling are high thermaldiffusivity gases, like hydrogen and helium. Cooling using ambienthydrogen provides a natural fit for twist-spun muscles that are poweredby hydrogen/air mixture, since injection of hydrogen during the coolingpart of the actuation cycle accelerates cooling, in preparation withsubsequent mixture of the hydrogen with air (or oxygen) to provideforward actuation.

Hybrid nanofibers yarn muscles and twist-inserted polymer fiber musclesare useful as intelligent sensors that detect environmental conditionsand provide either a reversible or non-reversible tensile or rotaryresponse (depending upon the design). One example is thereversibly-actuating hydrogen-sensing actuator of Example 14.Biologically functionalized guests in biscrolled yarns can respond toanalytes for self-powered sensing and/or control purposes. Choice of theguest within hybrid nanofiber muscles can be designed to provide eitherreversible actuation or the irreversible actuation needed for actuatingsensors that integrate exposure, like integrated time-temperatureexposure or integrated exposures to radiation or chemicals in theenvironment. Since the range of possible guests to meet particularsensor needs is enormous, it is possible to optimize performance forboth reversible and irreversible (i.e., integrating) sensing thatprovides mechanical actuation. Relevant for choice of guest, theliterature describes a host of materials that undergo either reversibleor irreversible volume changes as a function of temperature ortemperature history or chemical or radiation exposures.

Irreversible time-temperature-dependent actuation of polymer fiber andnanofiber hybrid yarn muscles can be used to hinder the opening ofpackaging for products that have been thermally over-exposed, likevaccines and other pharmaceutical products. Yarn or fiber volumeexpansion can be used to provide the friction that hinders opening ofthe container top of a pharmaceutical product. Alternatively, thisactuation can release a pin, which interferes with the opening of atwist top.

The coiled polymer fiber muscles and nanofiber yarn muscles can be usedfor the actuation of smart packaging materials, like cardboard boxesthat open and close porosity in response to ambient temperature (so asto regulate the temperature of products).

The ability of coiled neat carbon nanotube yarns to provide ademonstrated 7.3% reversible contraction (Example 5 and FIG. 2B) uponelectrothermal heating from room temperature to an incandescenttemperature (˜2,560° C.) in an inert atmosphere indicates the utility ofthese muscles to perform at temperatures where no other high strengthmuscle can operate. These muscles can be self-powered by changes inambient temperature to directly act as an intelligent material toprovide actuation, such as opening or closing a valve. As shown inExample 10, Example 11, and FIG. 8, changes in temperature can alsoprovide torsional actuation for neat carbon nanotube yarns (up to 30°/mmduring static measurements in vacuum for the SZ-ZS yarn in Example 11),which is useful for both temperature measurement and control purposes.Electrical pulse measurements to incandescent temperatures for neattwo-ply carbon nanotube yarn in vacuum provided 27°/mm rotation and anaverage rotational speed of 510 revolutions per minute (Example 11).

Though the achievable muscle torsional and tensile strokes will usuallybe relatively smaller, the dimensional changes of the coiled neat yarnsand the twist-inserted polymer fibers at below room temperature can beused to both indicate temperature and directly provide a functionalresponse, like opening or closing a valve. This valve control can beobtained in any temperature range by using the nanofiber yarn muscles orpolymer fiber muscles to provide tensile actuation, torsional actuation,or a combination thereof.

Instead on using changes in ambient temperature, heating due toabsorption of radiation (and especially light) can provide torsionalactuation of twisted polymer fiber muscles or twist-spun nanofiber yarnmuscles. Since carbon nanotubes are nearly perfect black body absorbers,optical heating readily occurs. Example 9 demonstrated a reversibletorsional actuation of 12.6°/mm when a half-wax-infiltrated Fermat yarnwas actuated by heating using a light pulse from a 100 W incandescentlamp. Reversible torsional and tensile photoactuation can also beobtained by reversible photoreaction of either nanofiber yarn host orguest (or a combination thereof) within a hybrid twist-spun nanofiberyarn muscle or the polymer in twist-inserted polymer fiber muscles.Reversible tensile and torsional photoactuation (using photo-thermalheating, photoreaction, or a combination thereof) can be used forautomatically controlling solar lighting for such spaces as homes andoffices, green houses, and solar cell farms. Most simply, reversibletensile actuation of hybrid nanofiber yarn muscles or twist-insertedpolymer fiber muscles can be used to open and close window blinds orshutters. Since a thermally powered muscle can span nearly the entirewidow dimension, and stroke can be amplified using pulleys, smalltemperature changes can be used to progressively open and close blindsand shutters without making any sound or consuming electrical energy,which is not the case for presently used expensive motors that open andclose blinds and shutters. Additionally, such photo-mechanical actuationcan be used to provide actuation for micro- and macro-optical devices.

Thermal-actuation through changes in ambient temperature,photo-actuation by changes in light exposure, and chemo-mechanicalactuation using changes in ambient chemical environment (includingambient moisture) can be used to generate mechanical energy by nanofiberyarn or twist-inserted polymer fiber actuation. This mechanical energycan be harvested as electrical energy by using such means as causing therotation of a permanent magnet in an arrangement of electrical coils orthe application of muscle generated strains to piezoelectric orferroelectric elements. This harvested energy can be used for example,for powering wireless sensors.

Since polymer fiber and hybrid yarn muscles can be highly absorbing inthe near infrared region where light is transported through tissue andblood (such as by incorporation of carbon nanotubes), this energyharvesting could be accomplished in the human body as long asoverheating of blood and tissue are avoided (such as by separating theenergy harvester from blood and tissue, but still using bloodcirculation for cooling).

Actuation of nanofiber yarns within clothing can provide variableporosity, wherein expanding or contracting guest materials would alteryarn length, yarn diameter, and/or coil diameter to open or closetextile pores, thereby increasing comfort or providing protectionagainst chemical or thermal threats. This opening and closing of textileporosity can also be accomplished by the rotation of micron-size paddlesattached to the actuating yarn and embedded in the textile, so thatpaddles are either parallel-to or perpendicular-to the textile surface.Since the teachings of invention embodiments provides nanofiber hybridyarns and twist-inserted polymer fibers that either expand or contractwith temperature increase, textile design using conventionally availableweaves is facilitated, so that the textile either opens porosity orcloses with temperature increase. The use of suitably heat-set coiledpolymer fiber muscles in which inserted fiber twist is in oppositedirection than that in the coil is the most convenient means to providefibers that increase length when heated, since achieving suchperformance for hybrid nanofiber yarns requires either the use of guestshaving a large negative volumetric expansion coefficient (which is rarefor materials) or the use of opposite direction twist insertion fornanofiber twist and coiling and the use of a frozen-in guest structurethat avoids irreversible actuation by partial cancellation of hybridnanofiber twist and the opposite twist due to nanofiber yarn coiling.Guest materials like paraffin wax can simultaneously provide the wellknown function of moderating against temperature change by absorbingenergy when temperatures become too hot and releasing this energy whentemperatures are decreased.

Nanofiber yarn muscles and twisted polymer fiber muscles in textiles canbe either actuated as a result of changes in ambient temperature,changes in chemicals in the environment, or by exposure to light orother radiations (so they are self-powered) or they can be actuatedelectrically by resistive heating. Such electrically powered actuationof yarns in textiles can be optionally controlled by detecting anambient condition (such as too high a temperature for fire fighters oran unsafe chemical or biological environment). While shape memory wiremuscles can be deployed to provide tensile contraction in textiles,their cost and uncomfortable feel has limited realization of textilesand other woven structures. In contrast, spools of both conductive andnon-conductive nylon are cheaply obtainable, widely used in clothing,and easily processed by invention embodiments into high strokeartificial muscles that provide either a contraction or an expansionupon actuation.

Twist-spun hybrid yarn muscles or twist-inserted polymer fibers can bewoven or sewn into textiles for actuator applications. The nanofiberyarn or twisted polymer fiber muscles can constitute either a majorityor a minority of yarns or fibers in the textiles. Depending upon thedesired anisotropy of actuation in the textile, these actuating yarns orfibers can be located largely in one textile direction (like a weft orwarp directions of a plain weave textile) or in all yarn and fiberdirections. Different twist-spun yarns or twist-inserted polymer fiberscan be optionally deployed in a textile for such purposes as providingto the textile the ability to actuate in response to exposure todiffering agents in the environment, light, or different ambienttemperatures.

This actuation of nanofiber yarn or polymer fiber muscles in textilescan be used to reversibly or irreversibly open or close textile pores inresponse to environmental thermal or chemical conditions, therebyenabling clothing derived from these textiles to be used dynamically toenhance wearer comfort or safety. Since the warp direction fiber in aplain weave textile can be essentially straight, while the weftdirection fiber goes up and down (above and under the weft fiber), thedeployment of the actuating fiber in the warp direction can be preferredwhen the goal is to use the actuating fiber array to do mechanical work(while at the same time benefiting from the high yarn cooling rate thatcan result from deployment of parallel thermally actuating fibers thatminimally thermally interact together). On the other hand, deployment ofactuating polymer fibers or nanofiber yarns in both weft and warpdirections of a plain weave textile (and for the diversity of directionsfor more complicated weaves, like for three-dimensional textiles) can beusefully deployed for such purposed as making textiles that changeporosity in response to thermal or chemical environmental conditions. Inaddition to use in clothing, textiles that change porosity in responseto temperature can be used for comfort adjusting tents.

The dimensional changes caused by polymer fiber or nanofiber yarn muscleactuation can be deployed in close-fitting garments to enable conveniententry to these garments, whether close-fitting clothing, a suite forunderwater diving, or a space suite. Various mechanisms can be deployedfor such purposes, such as photo-expansion (which later relaxes when thesuit is entered) or thermal contraction when the suite reaches body orother use temperature.

Actuating twist-spun nanofiber yarns and twist-inserted polymer fiberscan also be braided, which can serve the useful purpose of increasingthermal contact with surrounding liquids, surrounding ambient air, orother surrounding gas. Such increased thermal contact can be used toincrease the rate of nanofiber yarn or polymer fiber cooling to reverseactuation. Additionally, hollow braided twist-spun nanofiber yarn musclestructures and twist-inserted polymer fiber muscle structures can beusefully deployed to obtain accelerated cooling that reverses thermalactuation. One means to accomplish this is to pass cooling liquid or gasthrough the core of the braided structure. Another means is to use thecore of the hollow braid as a heat pipe—evaporation of a liquid fromcontact with the heated muscle to a liquid condensation site on the heatpipe will accelerate reversal of actuation. In fact, the hollow braidcan comprise segments that are independently actuated. In this case,transfer of heat from one braid segment during reversal of actuation toanother braid segment that is about to be electro-thermally actuated canreduce the electrical energy needed to actuate the second braid segment.Wicking means are needed for the operation of these heat pipes, whichcan be provided by either a porous, wet-able material in part of thebraid core or the location of grooves within the hollow braid that canprovide wicking.

Additionally, hollow braid structures can be deployed for fuel-poweredactuation of twist-spun yarns in the braid. In this case the fuel andoxidant (like air and either hydrogen or methanol) are transported tothe braid core, where the catalyst used for combustion is located.Alternatively, the optionally catalyzed exothermic chemicaltransformation of single chemical component or multiple chemicalcomponents that are optionally non-interacting or minimally interactingcan power chemo-thermal actuation. More generally, an artificial muscleyarn or a twist-inserted polymer fiber muscle can be actuated using afuel, whether or not it is acting individually or as part of an array,such as in a woven, knitted, or braided structure.

The polymer fiber muscle and hybrid yarn muscles can be usefullydeployed to provide attractive facial gestures for humanoid robots, suchas those used for companion robots for the elderly. Thirty facialmuscles are required to express ordinary human emotions (happiness,surprise, sadness, fright, etc.) and present motors used for roboticscannot do the job.

More generally, there are over 630 muscles in the human body andpresently used motors or hydraulic devices cannot provide theirfunctionality for either humanoid robots or prosthetic devices. On theother hand, the present coiled polymer fiber and nanofiber yarn musclescan be as be as thin as a human hair, can provide giant strokes, and canbe easily arrayed in parallel to do work. When using very thickthermally powered polymer muscle fibers or nanofiber yarn hybridmuscles, problems in obtainable fast cooling rates can be addressed byusing many fibers in an array and selectively actuating only some ofthese fibers, while others are allowed to naturally cool. Alternatively,surrounding fluid (like water) or a gas (like helium) can be used toaccelerate cooling rate (as can active cooling).

As another application embodiment, polymer muscle fibers or nanofiberyarn hybrid muscles in tight fitting clothing can be used like anexoskeleton to provide an amplified mechanical response to the feeblemovements of the infirm.

While many applications require highly reversible actuator responses,other applications exploit irreversible tensile or torsional actuation.One example of the latter is time-temperature indicators used to monitorthe thermal exposure conditions of perishable products. By matching theirreversible time-temperature response characteristics of an indicatormaterial to the degradation rate of a perishable product that is in thesame thermal environment, time-temperature indicators can be used tosignal that a perishable product (such as a vaccine) has been exposed toa thermal history that results in unacceptable product degradation.Irreversible muscles can use the irreversible dimensional changes ofguest material in a nanofiber yarn to provide an irreversible torsionalrotation or tensile actuation response. For instance, the tensileactuator response of a hybrid yarn can deflect the hand of a thermalexposure clock to indicate the severity of thermal exposure on productquality (such as denoting the remaining usable life of a vaccine).Alternatively, the yarn-driven torsional rotation of a pointer canprovide the indicator response. A “use or don't use” response can beprovided in this case by confining an indicating paddle so that onepaddle side (with one color) is visible until a critical torque resultsfrom guest dimensional change, which flips the paddle so that adifferently colored paddle side is visible. Since the kinetics ofpolymerization of thermally polymerizable diacetylenes match those ofmany important perishable products (like vaccines) [U.S. Pat. No.3,999,946] and since selected diacetylenes can provide large dimensionalchanges during polymerization (4.9% in the polymerization direction forthe symmetric diacetylene with substituent groups —CH₂OSO₂C₆H₅CH₃) [R.H. Baughman, J. Chem. Phys. 68, 3110-3117 (1978)]), these diacetylenesare useful as guests for these yarn-based time-temperature indicatordevices.

Since the quality of many types or products depends on whether or notthe product has been frozen (and the number of times that a product hasbeen frozen) hybrid nanofibers yarn muscles or twist-inserted polymerfibers can be similarly deployed as self-powered freeze indicators. Forexample, using a paraffin-wax-infiltrated nanotube yarn to providereversible actuation (at a temperature selected by choice of theparaffin) the arm of a “freeze clock” can count the number offreeze-defrost exposures of a product.

Changes in yarn resistance during actuation can be used to provide anadditional mechanism for providing an indicator response, as well asmore generically providing a means for the control of stroke forreversible actuators. More specifically, for time-temperature historyapplications or sensing temperature or chemicals, irreversibleresistance changes of actuated hybrid nanofiber yarns can be used inRFID (radio frequency identification) tags to provide a remotelyelectronically readable indicator response. Alternatively, tensile ortorsional actuation can be used to change the resistance or capacitanceof the antenna on a RFID tag, to thereby provide a remotely readableresponse.

Like for the case of the hybrid nanofiber yarn muscles, twist-insertedpolymer fibers can provide irreversible tensile or torsional actuationthat is useful for integrating time-temperature indicator devices ordevices that integrate chemical or radiation exposures and therebyprovide an irreversible mechanical response. To realize suchapplications, the polymer in the twist-inserted polymer fiber can bechosen to be unstable in the thermal, irradiation, or chemical exposureconditions that are of indicator interest. For example, polymerscontaining conjugated diacetylene groups are known that furtherpolymerize by 1,4-addition reaction when exposed to either suitably hightemperatures or actinic radiation (like UV, γ-ray, x-ray, or ionizingparticles). The dimensional changes associated with this reaction canprovide the needed irreversible indicator response, which can be eithertensile actuation or actuation by torsional rotation.

More generally, twist can be inserted in a polymer fiber at temperaturesthat are much lower that the temperatures that are of interest formonitoring in a time-temperature indicator. If muscle training thatprovides reversible muscle thermal response is avoided (like annealingof fiber mechanical strains by heating to the temperatures of interestfor time-temperature monitoring), subsequent exposure of the yarn tothese temperatures can provide the irreversible actuation that providesthe indicator response. Likewise, for integrated detection of chemicalsin the environment that react with multiple bonds, the twist-insertedpolymer fiber can be chosen that contains such bonds (like olefinic andacetylenic groups). Since coiling of nanofiber yarn and twist-insertedpolymer fiber can amplify both reversible and irreversible tensileactuation, such coiled nanofibers and twist-inserted polymer fibers arepreferred for irreversible exposure-integrating muscles used to providetensile actuation. One end tethering for a twisted nanofiber yarn muscleor a twist-inserted polymer fiber muscle that provides reversibletorsional actuation only when two end tethered is an especiallyconvenient means for providing an exposure integrating muscle thatprovides a torsional indicator response.

Yarn thermal actuators can be fuel powered, like for previousfuel-powered shape memory metal actuators that were capable of onlytensile actuation [“Fuel Powered Actuators and Methods of Using Same”,R. H. Baughman, V. H. Ebron, Z. Yang, D. J. Seyer, M. Kozlov, J. Oh, H.Xie, J. Razal, J. P. Ferraris, A. G. MacDiarmid, W. A. Macaulay, U.S.Pat. No. 8,096,119 B2]. One invention embodiment for providingfuel-powered twist-spun hybrid muscles is to provide a yarn thatcontains volume-changing guest in the yarn core and catalytic particles(like Pt or a Pt alloy) in the yarn shell for generating heat bycombining fuel and oxidant (like hydrogen or methanol with air). Thebiscrolling process [M. D. Lima et al., Science 331, 51-55 (2011)] canbe conveniently used for providing twist-spun muscles having such asheath-core structure. This is accomplished by depositing thevolume-changing host material on one extended area side of a forestdrawn sheet or sheet stack, and the catalyst particles on the remainingsheet area, and then asymmetrically inserting twist so that the catalystis in the yarn sheath and the volume-changing guest is in the yarn core.Similarly, such asymmetric deposition of volume-changing guest andcatalyst can be deposited in the sheet wedge that is formed during twistinsertion during direct spinning of a nanotube yarn from a nanotubeforest. As shown in Example 25, chemically driven reversible actuationcan be changed to irreversible actuation by selecting a muscleconfiguration and/or muscle type that provides irreversible actuation.The configuration used in Example 25 to provide irreversible actuationfor a single-ply, coiled nylon muscle is one-end tethering.

Prior-art work [J. Foroughi et al., Science 334, 494 (2011)] suggestedthat the previously described electrochemical nanotube yarn musclescould be driven in reverse to convert torsional mechanical energy toelectrical energy. Instead of using an electrochemical process toconvert mechanical energy to electrical energy, a quite differentapproach is used in invention embodiments for hybrid nanofiber yarn andpolymer fiber artificial muscles. The applicants harvest energyassociated with fluctuations in ambient temperature, in incident lightintensity, or in chemical environment (including humidity) to causetorsional and or tensile actuation of hybrid nanofiber yarn or polymerfiber artificial muscles. They then harvest this mechanical energy aselectrical energy by such processes as using tensile or torsionalactuation to deform a piezoelectric or ferroelectric that has contactingelectrodes, to deform an electric-field-biased,elastomeric-material-based capacitor, or by using the torsionalactuation of the artificial muscle to rotate a permanent magnet relativeto surrounding electric coils (or the inverse process where electriccoils are rotated with respect to a static magnetic field). Since meansfor converting mechanical energy to electrical energy are diverse andwell known, the novelty of the present technology is in the use hybridtwist-spun nanofiber yarn or twisted polymer fiber artificial muscles togenerate mechanical energy from these fluctuations in the ambient.

Actuation provided by twist-spun yarns and twisted polymer fibers can beused to affect the thermal expansion and compliance of yarn and polymerfiber assemblies, such as yarn and fiber tows and ropes, and otherstructures. For instance, in many applications it is desirable tocompensate the positive thermal expansion of one structural materialwith the negative thermal expansion of another structural material, Thepresent discovery that inserting coiling enables the negative thermalexpansion of neat twist-spun nanofiber yarns to be controllablyincreased in magnitude by a factor of about 10 or higher enables thematching of positive and negative thermal, so that desired thermalexpansion can be obtained (including one that is near zero). Sinceinfiltration of a material like paraffin into a twist-spun yarn can makethermal expansion even more negative, hybrid twist-spun yarns are alsoespecially useful for this application. Moreover, since twist-insertedpolymer fibers provide large controllable negative thermal expansion,which can be tuned and greatly enhanced by yarn coiling, thesetwist-inserted polymer fiber yarns are especially useful for suchapplication in which a near-zero thermal expansion is sought for fiberassemblies (and yarn and fiber assemblies) or for composites.

Melting of solid guest within a twist-spun hybrid yarn causes yarntensile modulus to reversibly decrease (and, correspondingly, the yarncompliance to increase). Hence, this type of actuation can be used forvarious purposes, like varying the stiffness of wings on a micro-airvehicle or the stiffness of an artificial muscle.

More generally, combinations of different types of actuating nanofiberhybrid yarns and different types of twist-inserted polymer fibers can becombined for in-parallel operation or operation within a given actuatorlength to provide engineered responses in terms of overall stroke andcompliance. In the case of electro-thermal actuation, independentelectrical connections to different segments in an actuator length orfor different in-parallel actuator lengths can be used to tune strokeand compliance according to application needs, such as for a humanoidrobot that is catching a ball. Such combinations also enable responsetuning for environmentally powered artificial muscles.

The observed low degree of hysteresis in muscle stroke when thenanofiber yarn and polymer fiber muscles are actuated under fixed load,which contrasts with that for shape memory metal muscles, facilitatescontrol of muscle stroke. This is demonstrated in Example 23, were aconventional proportional-integral-derivative controller (PIDcontroller) is used to minimize positioning error. This low or absenthysteresis is used in Example 33 to provide two-dimensional displacementof a load by using an arrangement of non-parallel coiled polymer fiberor hybrid nanofiber yarn muscles or muscle segments.

By using an electrical heating element for electrothermal actuation thatis mechanically coupled to a hybrid nanofiber yarn muscles or a polymerfiber muscle, the resistance of the heating element can be deployed fordetermining muscle stroke by measuring the resistance of this heatingelement. For such applications, the resistance of the electrical heatingelement is preferably insensitive to temperature, but sensitive tomuscle strain. An example of such a suitable electrical heating elementis a carbon nanotube sheet that is wrapped around a polymer fiber muscleor a hybrid nanofiber yarn muscle. Similarly, such a wrapped carbonnanotube sheet or like element can be used to measure muscle stroke fornon-electrically powered muscle actuation, such as that provided byphoto-thermally or chemically powered actuation or powered by changes inambient temperature or chemical environment (including environmentalhumidity).

Polymer fiber muscles or nanofiber yarn muscles of invention embodimentscan also be used to thermoacoustically generate sound when drivenelectrically or photo-thermally, as shown in Example 24. For suchapplication, the sound generating element (such as a carbon nanotubesheet wrapped about a polymer fiber muscle, a hybrid nanofiber yarnmuscle, or a neat nanofiber yarn muscle) should have high porosity andsufficiently low thermal inertia that it can heat and cool at acousticfrequencies. This sound can be optionally detected so as to provide ameans to sense muscle activity. As for the case of Example 24, thefrequency at which the nanofiber conducting layer is driven can be muchhigher than the frequency at which a nanofiber-wrapped, twist-insertedpolymer fiber muscle is electrothermally actuated, since the nanofiberlayer can heat and cool much faster than can the typically much moremassive underlying polymer fiber muscle. For example, instead ofapplying a DC square-wave voltage pulse, an audio signal voltage at muchhigher frequencies (but with similar root-mean-square amplitude) can beapplied to produce the Joule heating needed for electrothermal muscleactuation. Taking into account for acoustic signal processing thefrequency doubling that occurs if the nanofiber sheet is initiallyunheated and the rising temperature of this sheet during muscleactuation, a muscle-supported nanofiber sheet could broadcast anacoustic message (saving, for instance: “The muscle is actuated”).

Additionally, elements having the configuration of a polymer fiber ornanofiber yarn muscles can be used to generate sound even when notdeployed as muscles. Such thermoacoustically sound generating fiber oryarn elements can be operated in-parallel or non-parallel yarn orpolymer arrays to provide increased sound intensity, and to providedirectional steering of sound generation (by varying the relative phasesof the electrical signal inputted to the different yarns or fibers in anarray).

Such sound generating fiber or yarn elements can also be used forcancellation of ambient noise when operated out-of-phase with respect tothis ambient noise.

A direct current bias can be optionally applied to thesethermal-acoustic nanofiber yarn or polymer fiber elements, so that soundgeneration is at the same frequency as an input alternatingcurrent—thereby avoiding the doubling of output sound frequency withrespect to the input frequency that occurs when no dc bias is applied.Even when the function of interest is exclusively mechanical actuation,such direct current bias can be usefully applied to provide oscillationof actuator stroke about a desired actuator stroke.

As a further application embodiment, the tensile or torsional actuationof twisted polymer fiber muscles and twist-spun nanofiber yarn musclescan be used to cancel unwanted translational or rotary movements. Thisembodiment operates by the application of muscle generated torque ortensile force that counteracts the unwanted translational or rotarymovements.

Additional information of the present invention is included in M. D.Lima et al., “Electrically, Chemically, and Photonically PoweredTorsional and Tensile Actuation of Hybrid Carbon Nanotube Yarn Muscles,”Science, 338, 928-932 (2012), which is incorporated herein by referencein its entirety.

EXAMPLES

The examples provided herein are to more fully illustrate some of theembodiments of the present invention. It should be appreciated by thoseof skill in the art that the techniques disclosed in the examples whichfollow represent techniques discovered by the Applicant to function wellin the practice of the invention, and thus can be considered toconstitute exemplary modes for its practice. However, those of skill inthe art should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments that are disclosed andstill obtain a like or similar result without departing from the spiritand scope of the invention.

Example 1

This Example 1 described the fabrication of yarn muscles containingparaffin wax guest. Though we obtained similar results for othercommercially obtained waxes (like those used for canning and candles),unless otherwise indicated the described results are for a wax morelikely to be readily available to future researchers (Sigma-Aldrich411671 wax), which comprises a mixture of alkanes. Results in Section 3show that this wax fully melts at ˜83° C., expands by ˜20% between 30and 90° C. during solid-state transitions and melting, and provides ˜10%additional volume expansion between 90 and 210° C.

MWNT yarns were typically infiltrated with paraffin wax using the “hotwire method”, wherein a two-end-tethered, twist-spun yarn, underconstant tensile load was electrically heated to above the melting pointof the paraffin wax and then contacted with a small amount of solidparaffin. Upon touching the heated yarn with flakes of solid paraffin ordroplets of molten paraffin, the paraffin quickly spread through theyarn. For a 100 μm diameter MWNT yarn, an applied voltage of about 3V/cm was sufficient to enable infiltration of the Aldrich paraffin wax.Since excess paraffin on the yarn surface degraded actuation, the yarnwas electrically heated to above the evaporation temperature of theparaffin (˜233° C.) until no excess paraffin was observed on the yarnsurface. The need for this second step can be avoided by multipleapplications of molten droplets to the heated yarn, and stopping thisprocess before excess paraffin accumulates on the yarn surface. Anotherwax infiltration method, which was used for all Fermat yarns that weredirectly twist spun during forest draw, is to slowly immerse atwo-end-tethered, as-spun yarn into melted paraffin (about 0.1 cm/s)under constant tensile load (˜10% of the failure stress). SEM microscopyof the yarn cross-section indicate that porosity of the neat yarn hasbeen largely eliminated by wax infiltration using the above slowimmersion method.

Example 2

This Example 2 described the fabrication of yarn muscles containingpolydiacetylene guest. The utilized diacetylene (DA) was10,12-pentacosadiynoic acid [CH₃(CH₂)₁₁C≡C—C≡C(CH₂)₈COOH], which waspurchased from Alfa Aesar Co., Ltd and used as received. An as-spun,two-end-tethered, Fermat yarn (9 μm in diameter, with 20,000 turns/m ofinserted twist) was first immersed in 8 M DA tetrahydrofuran solutionfor an hour, and then the DA infiltrated yarn was removed from thesolution and dried overnight at room temperature while maintainingtethering. UV light (254 nm from a 30 W UV lamp) was used to in situpolymerize the DA into a polydiacetylene (PDA). The polymerization timewas typically around 3 minutes, which caused the yarn to develop a darkblue color. However, polymerization was incomplete, in part becausenanotube and diacetylene absorption prevents deep penetration of the UVlight inside the yarn.

Example 3

This Example 3 described the fabrication of a hybrid yarn musclescontaining polyethylene glycol guest. Polyethylene glycol (PEG),H(OCH₂CH₂)_(n)OH, with an average molecular weight of ˜6000 and amelting temperature range of 60 to 63° C., was obtained as flakes fromSigma Aldrich (Bio-Ultra 6000) and used as received. The PEG wasinfiltrated into the lower half of a 13 μm diameter Fermat yarn(containing 15,000 turns/m of inserted twist) by immersion of this yarnsegment in a molten bath of PEG for 30 minutes at about 100° C. Then thetwo-end-tethered yarn was removed from the PEG bath and allowed to coolto room temperature. The diameter of the PEG-filled yarn segment was 17μm and the bias angle was 31°.

Example 4

This Example 4 described the fabrication of yarn muscles containingpalladium guest. Using e-beam deposition (CHA-50 e-beam evaporator),individual nanotubes and nanotube bundles within a stack of twoco-oriented MWNT sheets, supported by rigid rods, were coated first witha ˜5 nm thick Ti buffer layer (to ensure uniform Pd deposition) and thenwith a 60, 80, 120, or 140 nm thick Pd layer, where the layerthicknesses are nominal values that correspond to the layer thickness ofdepositions on a planar substrate that is in the same environment. Then,the sheets stack was twist spun (100 to 200 turns/m) to obtain a 144 μmdiameter yarn having dual-Archimedean structure. A 60 nm Pd layer wassufficient to obtain reversible actuation of the yarn and a thickercoating undesirably increased the difficulty of inserting yarn twist.

Example 5

This Example 5 showed that yarn coiling dramatically increases tensileactuation for both neat and paraffin wax infiltrated carbon nanotubeyarns. Tensile contraction versus temperature for coileddual-Archimedean yarn, before and after wax infiltration, is compared inFIG. 2A with corresponding data (figure inset) for non-coiled Fermatyarn. Wax infiltration greatly enhanced tensile contraction for allyarns, as did yarn coiling. Despite a difference in the load dependenceof actuation, similar tensile strokes were obtained for non-coiled,Fermat and dual-Archimedean yarns having similar diameter and twistangle (FIG. 6). Heating the neat coiled yarn from ambient toincandescent temperature (about 2,560° C.) under 3.8 MPa tensile stressprovided a reversible yarn contraction of 7.3% (FIG. 2B), correspondingto 0.16 kJ/kg work capability. Since yarn coiling greatly enhancedtensile actuation stroke, coiled yarns (FIG. 1E) are the focus of mostof the below studies on tensile actuation.

Example 6

In this Example 6, tensile actuation at a remarkable 1,200 cycles perminute and 3% stroke was demonstrated for over 1.4 million cycles (FIG.3A) using a two-end-tethered, paraffin-wax-filled, coiled Fermat yarnthat lifted 17,700 times its own weight. This high-rate was produced byapplying a 20 Hz, 18.3 V/cm square wave voltage at 50% duty to a 3.8 cmlong yarn weighing 2.25 μg/cm. Fast passive cooling in 25 ms resultedfrom the small yarn and coil diameters (11.5 μm and 20 μm,respectively). Applying well-separated 25 ms pulses yielded 1.58%initial contraction and 0.104 kJ/kg of mechanical energy during thiscontraction at an average power output of 4.2 kW/kg (four times thepower-to-weight ratio of common internal combustion engines).

Example 7

This Example 7 showed that the performance of the yarn of Example 6 as atensile actuator can be optimized by increasing the applied voltage andmechanical load, while reducing the pulse duration. FIG. 3B shows aseries of actuations wherein the yarn lifts 175,000 times its mass in 30ms when 32 V/cm is applied for 15 ms. The work during contraction (0.836kJ/kg) provided a power output of 27.9 kW/kg, which is 85 times the peakoutput of mammalian skeletal muscles (0.323 kW/kg) and 30 times themaximum measured power density of previous carbon nanotube muscles [J.Foroughi et al., Science 334, 494 (2011)]. However, the high appliedelectrical power reduces cycle life by causing excessive heating andslow paraffin evaporation.

Actuator stroke and work capacity during contraction can beindependently maximized by optimizing the applied load, though ingeneral they cannot be simultaneously maximized. FIG. 3C shows thestress dependence of actuator stroke and work capacity for differentamounts of twist insertion in a wax-infiltrated, 150 μm diameter,dual-Archimedean yarn that is two-end tethered. Reversible contraction,which is greatly enhanced for yarn having sufficient twist to causecoiling, resulted from steady-state electrical heating to just below thewax vaporization temperature. Applying high stress decreases stroke, dueto the yarn's lower Young's modulus in the contracted state (containingmolten wax) and correspondingly larger elastic elongation under loadthan the initial state (where the solid wax provides structuralreinforcement for both tensile and torsional deformations). The strokefor highly coiled yarn decreases at low stresses (FIG. 3C), which isconsistent with the close proximity of adjacent coils hinderingcontraction.

FIG. 3C showed that there is an optimal amount of coiling that maximizeseither stroke or work during contraction for the wax hybrid yarn. Amaximum contraction of 5.6% was observed at 5.7 MPa stress for a coiledFermat yarn having intermediate twist. Adding 6.8% more twist to thecoiled yarn increased the stress of maximum contraction (16.4 MPa for5.1% strain) and the maximum measured contractile work (1.36 kJ/kg at 84MPa), which is 29 times the work capacity of natural muscle.Subsequently reducing twist by 41% eliminated coiling and reducedmaximum contraction and contractile work to low values (0.7% and 0.31kJ/kg, respectively). Contractions of 10% under 5.5 MPa stress wererealized for a 150 μm diameter, partially coiled, dual-Archimedean yarnby applying well-separated 50 ms, 15 V/cm pulses (FIG. 3D). Since thecross-sectional area of this yarn was 170 times higher than for the yarnof FIG. 3A and FIG. 3B, passive cooling in ambient air was lesseffective: the cooling time increased from about 25 ms to about 2.5 s,resulting in a low contractile power density when both heating andcooling times are considered (0.12 kW/kg).

Example 8

In this Example 8, experimental data on tensile actuation versus twistinsertion for a neat Fermat yarn in the FIG. 1A configuration shows theimportance of twist and resulting bias angle increase on thermalcontraction (FIG. 5). With increase of inserted twist from approximately9,650 turns/m to approximately 28,130 turns/m, tensile actuation atconstant applied power increased about 2.8 times (from about 0.03% toabout 0.086%). However, when the start of coiling was first observed (at33,800 turns/m) there was about 4.5% decrease in thermal contraction,which might be due to the predominance of non-coiled yarn segments inproviding contraction when there is little coiling and the effect ofintroduced coiling on decreasing modulus.

Example 9

In this Example 9, very fast, highly reversible torsional actuation wasdemonstrated for two million cycles for a 6.9 cm long, 10 μm diameter,two-end-tethered, half-wax-infiltrated homochiral Fermat yarn thatrotated a paddle at yarn midpoint (FIG. 1B configuration). The hybridyarn accelerated a 16.5 times heavier paddle to a full-cycle-averaged11,500 rotations per minute—first in one direction and then in reverse(FIG. 4A). Even though actuation temperature was far above T_(mf), thishigh cycle life resulted because of the presence of a torsional returnspring (the unactuated yarn segment of FIG. 1B). FIG. 4B shows thedependence of torsional rotation on input electrical power and appliedtensile load for a similar yarn that rotated a 150 times heavier paddlefor a million highly reversible cycles. Increasing load increasedrotation speed from 5,500 revolutions/minute to a maximum of 7,900revolutions/minute. Reversible torsional actuation (12.6°/mm) was alsodriven for a half-wax-infiltrated yarn by replacing electrical heatingwith heating using light pulse from a 100 W incandescent lamp.

Example 10

This Example 10 characterizes the effect of wax infiltration ontorsional actuation for a two-end tethered homochiral yarn, whereinone-half of the yarn is actuated and the other half largely functions asa torsional return spring. The utilized 16 μm diameter Fermat yarn had15,000 turns/m of inserted twist and a bias angle of 35°. Theconfiguration for the wax containing yarn was exactly the same as forFIG. 1B, and that for the non-infiltrated yarn differs only in that thetwo yarn segments were equivalent except that electrical power wasapplied to only one-half of the yarn length. In these comparativeexamples the same mechanical load was applied and the voltage used toachieve actuation was identical (11.6 V/cm). Although some torsionalactuation rotation was observed for the neat yarn (4.9°/mm), which maybe due to small difference in torsional and tensile moduli between thelow and high temperature yarn segments, this rotation was low comparedto the 71.2°/mm torsional actuation observed when one of the yarnsegments was subsequently infiltrated with paraffin wax.

Example 11

This Example 11 demonstrated that use of two-ply heterochiral yarn(instead of a non-plied heterochiral yarn) enables reversibleelectrothermal torsional actuation for the FIG. 1D configuration. A SZyarn was obtained by inserting about 30% extra twist into an 11 μmdiameter, Fermat Z yarn having an initial twist of 20,000 turns permeter. This highly twisted yarn was then folded upon itself, so thatpart of the Z twist was converted to S twist due to plying. A ZS yarnwas made analogously. Then these yarns were knotted together, and apaddle was attached at the position of the knot. The resulting two-plySZ-ZS yarn structure was 20 μm in diameter.

Steady-state measurements of torsional actuation as a function of inputelectrical power measurements (FIG. 8) show that reversible torsionalrotation results in the FIG. 1D configuration for heterochiral, two-plyFermat yarn that is either (1) wax-filled and actuated to above themelting point of the wax or (2) neat and actuated to incandescenttemperature in vacuum. The applied stresses for these experiments were3.2 MPa for the neat yarn and 5.8 MPa for the wax-filled yarn. While themaximum torsional actuation achieved here for wax-filled SZ-ZS yarn(68°/mm) is about the same as for the half-infiltrated homochiral yarnof Example 10 in the FIG. 1B configuration (71.2°/mm), the neat SZ-ZSyarn in vacuum provided 30°/mm torsional actuation (versus the 4.9°/mmfor the half-actuated, homochiral yarn of Example 10 in air). Thislatter difference shows, at least in part, the actuation enhancement forneat yarn that results from actuation to high temperatures (which arepresently enabled at the same power as for wax-filled yarn by usingvacuum to eliminate convective energy loss for the porous neat yarn).Although low for nanotube torsional actuators, this 30°/mm of torsionalactuation for the neat yarn is 200 times the maximum previously reportedfor shape memory alloys, ferroelectric ceramics, or conducting polymers.Torsional actuation was investigated for this neat two-ply yarn whendriven in vacuum to incandescent temperatures using 9.7 V/cm voltagepulses with 1 Hz frequency and 20% duty cycle. A 27°/mm rotation wasobserved with an average speed of 510 revolutions per minute.

This reversible behavior contrasts with the lack of reversibility ofactuation of heterochiral, single-ply yarn in the FIG. 1D configurationwhen the yarn does not contain solid guest at all points in theactuation cycle. In the latter case, permanent cancellation of theopposite twist in the two yarn segments occurs during actuation, therebyresulting in permanent elongation and reduction of torsional rotationduring cycling.

Example 12

This Example 12 demonstrated that paraffin-wax-infiltrated, coiledcarbon nanotube yarn can generate giant specific torque and that thistorque can be used to hurl an object. The measured static specifictorque versus applied electrical power for a 100 μm diameter, 6.4 cmlong, fully-infiltrated, heterochiral, dual-Archimedean yarn havingapproximately 3,000 turns/m of inserted twist per stack length is shownin FIG. 4C. A maximum specific torque of 8.42 N·m/kg was generated forthis 100 μm diameter yarn, which is five times higher than demonstratedfor electrochemically driven nanotube yarns [J. Foroughi et al., Science334, 494-497 (2011)] and slightly higher than for large electric motors(up to 6 N m/kg). This torque was determined using a digitalmicrobalance to measure the force exerted by a metal paddle (23 mm long)attached to the central junction point of the heterochiral yarn. Thepaddle was in the horizontal position, pressing against the plate of themicrobalance during force measurement. So that no force was applied tothe microbalance when the actuating voltage was zero, at the beginningof the experiment the wax in the yarn was melted (by electrical heating)and re-solidified while the metal paddle was in contact with the plateof the microbalance. This same paraffin-wax-infiltrated heterochiralyarn was used to hurl a projectile by rotating the arm of a miniatureGreco-Roman style catapult (FIG. 4C, lower inset) by 300° Though themaximum torsional actuation temperature was above the temperature atwhich wax melting was complete, reversible operation of the catapult wasachieved.

Example 13

In this Example 13, Applicant has also demonstrated reversible,thermally powered torsional actuation for hybrid yarn containing othervolume-expanding guests. This is presently demonstrated forCH₃(CH₂)₁₁C≡C—C≡C(CH₂)₈COOH, which was infiltrated into twist-spunFermat yarn (d=9 μm and α=26°) and photopolymerized by 1,4-addition toproduce a polydiacetylene (PDA), as described in Example 2. Like for arelated polydiacetylene used to make color-changing carbon nanotubeyarns [H. Peng et al., Nature Nanotech. 4, 738 (2009)], the producedpolydiacetylene is polychromatic, providing a blue-to-red phasetransition at ˜57° C. that is reversible unless too high a temperatureis reached. However, the partial degree of polymerization is acomplication, since the unpolymerized monomer melts at ˜63° C., andproduces additional yarn expansion.

For the first investigated horizontal configuration (FIG. 7A, which isanalogous to the FIG. 1B configuration) the two-end-tethered homochiralFermat yarn supported a constant load (2 MPa, when normalized to thecross-section of the unactuated yarn). The PDA-containing yarn segmentused for torsional actuation was 3 cm long and the total yarn length was7 cm, of which 6 cm was located before the wire eye hole support and therest of the yarn length vertically supported a slotted weight, which wasnot free to rotate (but was free to move vertically as the actuatingyarn segment contracted and expanded). Hence, the non-infiltrated yarnlength acts as a torsional return spring. Both for this and the seconddescribed configuration, Joule heating was by applying a voltage betweenthe yarn end and the metal eye hole support. When 2 mA DC current wasapplied to the yarn (corresponding to 13 mW/cm input power), reversiblepaddle rotation of 100°/mm was produced as the actuated yarn untwistedduring Joule heating. Highly reversible actuation was demonstrated forover 5,000 on-off cycles, which were the maximum investigated.

Even when in a one-end-tethered configuration (FIG. 7B, which isanalogous to the FIG. 1C), the fully infiltrated, PDA-hybrid yarn canprovide reversible torsional rotation even when heated to above themelting point of unpolymerized monomer. When 2 mA DC current was appliedto the 3 cm long diacetylene-infiltrated yarn segment, the paddlerotated in the direction that corresponds to untwist of the actuatingyarn, and this rotation was then reversed when electrical heating wasstopped. This indicates that the polydiacetylene inside the yarnfunctions as an internal torsional spring to enable torsional actuationto reverse when yarn volume decreases during cooling. Since thecorresponding neat yarn does not have a return spring, it does notprovide reversible torsional actuation.

Due to a few percent volume increase at this blue-red phase transitionand a larger volume change from melting incompletely polymerized monomerat 63° C., reversible torsional rotation of 100°/mm was obtained foractuation to below 80° C. for the two-end-tethered, half-infiltratedyarn configuration of FIG. 1B. Actuation to higher temperatures waspoorly reversible, likely because of an irreversible phase transition.

Example 14

This Example 14 demonstrated actuation powered by absorption for thepalladium hybrid carbon nanotube yarn of Example 4. The configuration ofFIG. 1D was deployed for characterization of torsional actuation using0.022 MPa applied tensile stress. Reversible torsional actuation waspowered by the absorption and desorption of hydrogen on a 60 nm thickpalladium layer on nanotube bundles within a dual-Archimedean yarn.Since this 144 μm diameter yarn contained 90 wt % palladium, theresulting high torsional rigidity restricted twist insertion to up to200 turns/m. Nevertheless, a one-end-tethered yarn rotated at its freeend a thousand times heavier paddle during hydrogen absorption.Injection of 0.05 atm H₂ into a vacuum chamber containing the actuatorcaused 1.5 paddle rotations within ˜6 s, which was fully reversed on asimilar time scale during repeated cycling between hydrogen exposure andvacuum. Cantilever-based actuators exploiting the dimensional changes ofa 10 μm thick Pd alloy layer have been previously demonstrated [M.Mizumoto, T. Ohgai, A. Kagawa, J. of Alloys and Compounds 482, 416-419(2009)], but the response time was in tens of minutes. The yarn's100-fold faster response rate resulted from yarn porosity and thethinness of the Pd coating. Such yarn actuators might be used asintelligent muscles that rapidly close an inlet when a targeted hydrogenpressure is exceeded.

Example 15

This Example 15 demonstrated that liquid absorption and desorption canalso drive actuation, as shown in FIG. 9, where torsional actuation of atwo-end-tethered Fermat yarn is shown as a function of immersion lengthin liquid. Largely reversible torsional rotation was obtained by varyingthe immersion depth of a two-end-tethered homochiral yarn in a wettingliquid. The actuator test configuration of FIG. 9A was deployed, wherethe total yarn length was 80 mm, the paddle used for recording actuationwas approximately at yarn midpoint, the top end of the yarn was attachedto a flexible rod support and the bottom yarn end was rigidly attachedto the bottom of a stationary 20 mm diameter glass vial. Theinvestigated Fermat yarn contained approximately 25,000 turns/m ofinserted twist and the initial yarn diameter and bias angle were 8 μmand 32°, respectively. The Mylar paddle was 3.5 mm wide, 2 mm tall and0.1 μm thick and weighed 1.0 mg, which is ˜100 times heavier that thetotal yarn.

The actuator response was trained by first injecting about 4 cm³ of testliquid into the glass, which provided a yarn immersion depth of about 12mm. After the paddle rotation stopped, indicating torque balance, theliquid was removed at about 0.1 mL/s, which corresponded to a 0.3 mm/sdecrease in yarn immersion depth. The liquid filling/removal procedurewas repeated 3 times to ensure a degree of reversibility for thedependence of paddle rotation angle on yarn immersion. After thistraining period, the data in FIG. 9B shows that the paddle rotationangle (φ) is a function of yarn immersion depth, with approximate slopesof 49.6±3.4 and 35.3±1.7 degree/mm for acetonitrile and hexane,respectively.

Example 16

This Example 16 experimentally demonstrated torsional actuation for atwo-end-tethered, homochiral, non-coiled Fermat yarn that was partiallyinfiltrated with polyethylene glycol (PEG), using the method describedin Example 3. PEG was chosen as guest in the carbon nanotube yarn sinceit expands volume by 10% during melting [L. J. Ravin, T. Higuchi, J. Am.Pharm. Assoc. 46, 732 (1957)]. The yarn diameter, amount of insertedtwist, and bias angle were 17 μm, 15,000 turns/m, and 31°, respectively.The total yarn length was 5.2 cm, a 2.6 cm long section at one yarn endwas infiltrated with PEG, and the paddle was at the junction betweeninfiltrated and non-infiltrated yarn segments, like in FIG. 1B. Thispaddle, which is 92 times heavier than the infiltrated yarn segment, wasa rectangular Kapton tape strip (3.7 mm long, 1.1 mm wide, and 130 μmthick). Torsional actuation was recorded using a high speed movie camera(240 frames/s), and data was obtained by frame-by-frame analysis of thetime dependence of paddle rotation angle.

Using the FIG. 1B configuration, actuation to above the meltingtemperature of the PEG was produced by applying a 2.4 mA square wavecurrent pulse (3.4 Hz frequency and 25% duty cycle) along the entireyarn length. The corresponding power during actuation was 16 mW/cm andthe tensile stress applied during actuation was 23 MPa (when normalizedto the cross-section of the non-actuated yarn). Using this pulsedelectrical power input, a maximum rotation speed of 1,040 revolutionsper minute and a torsional rotation of 37°/mm were obtained (during anactuation cycle where the infiltrated yarn segment first untwists duringheating and then re-twists during unaided cooling). No degradation inactuation was observed up to the maximum number of observed cycles(100,000 cycles).

Example 17

As an alternative to electrical heating, this Example 17 demonstratedthat torsional and tensile actuation of paraffin-containing carbonnanotube hybrid yarns can be produced by incandescent heating from a 100W white-light lamp that was manually switched on (1.6-2 s) and off(0.3-0.5 s). Using the FIG. 1B configuration, reversible torsionalactuation of 12.6°/mm was obtained for a two-end-tethered homochiralFermat yarn (about 15 μm diameter with approximately 20,000 turns/m ofinserted twist) that was half-infiltrated with paraffin wax. Reversibletensile contraction occurred simultaneously with untwist of theparaffin-containing yarn segment during torsional actuation caused byphotonically heating this yarn segment.

Example 18

This Example 18 demonstrated the use of a lever arm to achieve highfrequency, large stroke, tensile actuation for a two-end-tethered,homochiral, coiled Fermat yarn that was fully infiltrated with paraffinwax (using the method described in Example 1). The yarn diameter withinthe coil and the coil diameter were about 22 μm and about 37 μm,respectively. The total yarn length was 7.3 cm when a stress of about15.2 MPa was applied. The rigid lever arm was a 50 mm long aluminum tube(0.159 cm outer diameter and 0.088 cm inner diameter) weighing 0.196 gthat was free to pivot about one rod end. The force generated by theactuating yarn was applied a distance of 3 mm from the pivot point(using an aluminum ring attached to yarn end) to provide a mechanicaladvantage of 0.06. Actuation to above the melting temperature of theparaffin was produced by applying about 13 V/cm voltage (at 5-10 Hzfrequency and 50% duty cycle) along the entire yarn length. Using thispulsed electrical power input, a maximum vertical displacement of 11.9mm was obtained for an applied frequency of 5 Hz. Applying the force ofactuation at 5 mm from the pivot point (corresponding to a mechanicaladvantage of 0.1) and using higher applied frequencies of 6.7 and 10 Hz,this displacement decreased to 10.4 mm and 3.5 mm, respectively.

Example 19

This Example 19 demonstrated amplified, high frequency tensile actuationfor a two-end-tethered, homochiral, coiled Fermat yarn that was fullyinfiltrated with paraffin wax. The yarn muscles and the yarnconfiguration (like in FIG. 1A) were the same as for Example 18. Likefor this example, one end of the actuating yarn was attached by knottingto an aluminum ring. However, in this Example 19 the force generated bythe yarn muscle was applied (via the attached aluminum ring) to a 254 μmdiameter and 60 mm long tempered wire cantilever that had a lineardensity of about 0.04 g/cm. Since the attachment point to the wire was20 mm from the cantilever base, the mechanical advantage was 0.33.Actuation to above the melting temperature of the paraffin wax in theyarn muscle was produced by applying about 13 V/cm voltage (at 75 Hzfrequency and 50% duty cycle) along the entire yarn length. Using thispulsed electrical power input, a maximum vertical displacement of thecantilever tip by 3.43 mm was obtained at 75 Hz.

Example 20

This Example 20 experimentally and theoretically relates the volumechange of a yarn guest to the volume change of an actuated yarn. Moviesrecorded by optical microscopy were used to characterize yarn structurechanges as overall yarn length change was measured during isotonicelectrothermal tensile actuation. A wax-filled, non-coiled,dual-Archimedean yarn was two-end-tethered in the FIG. 1A configuration.This 150 μm diameter yarn had a bias angle of about 35° and contained2,500 turns/m of inserted twist per stack length. The applied load was13.4 MPa and 123 mW/cm of electrical power was applied to cycle betweenactuated and non-actuated steady states. We observed that yarn diameterincreased by 4.06±1.87% as total yarn length contracted by 0.585±0.003%,thereby indicating that the yarn volume increased by 7.7±2.6% duringactuation. As expected because of nanotube volume, this percent yarnvolume change from ambient temperature to about 210° C. was much smallerthan the percent volume change of the wax, which is about 30%.

For comparison with the above result, the percent volume change for theyarn during actuation (ΔV_(y)/V_(y)) was calculated from the density ofbundled nanotubes (ρ_(b)), the initial density of the wax beforeactuation (ρ_(w)), the fraction of yarn weight that is wax (F_(w)), andthe fractional change in wax volume during actuation (ΔV_(w)/V_(w)). Theresult is:ΔV _(y) /V _(y)=(ΔV _(w) /V _(w))(1+(ρ_(w)/ρ_(b))((1−F _(w))/F _(w)))⁻¹.

A nanotube bundle density of ρ_(b)=1.65 g/cm³ was calculated byapproximating that a typical bundle contains hexagonally close-packed, 9nm diameter nanotubes having six walls. Using 0.9 g/cm³ for the densityof the solid wax, ρ_(w)/ρ_(b)=0.54. For the measured weight fraction ofwax in a 180 μm diameter, dual-Archimedean yarn (0.28) and the measuredΔV_(w)/V_(w) between 30 and 210° C. (30%), the calculated yarn volumechange is 12.6%, which is within two standard deviations of the abovemeasured value (7.7±2.6%).

Example 21

This Example 21 demonstrates the use of polymer-filled, coiled carbonnanotube yarns as tensile actuators, as well as the enhancement oftensile stroke by resin infiltration in a low-twist yarn state,subsequent polymerization of this resin and then insertion of the hightwist needed to produce yarn coiling. A 300 μm diameter hostdual-Archimedean carbon nanotube yarn was produced by inserting 100 to200 turns/m of twist (under 5 g load) into a stack of four forest-drawnMWNT sheets that were 2.5 cm wide and 15 cm long. This low-twist hostyarn was then infiltrated with silicone rubber resin (bi-componentSiLicone cps 1200 from Silicones, Inc.). After the silicon rubber wascured (for ˜24 hour at room temperature) the composite yarn was twistedunder about 0.2 MPa tension until it was fully coiled. The finaldiameter of the yarn was about 270 μm. This low-twist-infiltrationmethod allows a coiled hybrid yarn to be fabricated that contains a veryhigh weight and volume percent of guest material. For the abovedescribed silicone rubber/carbon nanotube yarn the weight percent ofsilicone rubber was about 95%. As a result of this high yarn loadingwith guest, the large thermal expansion of the guest, and thepost-infiltration use of coiling, giant tensile contraction resultedduring electrothermal actuation. Upon electrical heating (using 0.2second square-wave pulses of 5V/cm), the coiled hybrid nanotube yarnreversibly contracted by up to 35% under a stress of 5 MPa (FIG. 10).

Example 22

This Example 22 demonstrates the use of a single coiled, two-ply, SZtwisted, silver-coated nylon fiber as a noiseless actuator forcontrolling the opening of window shutters, which is pictured in FIGS.22C-D. The precursor non-twisted 200 um diameter silver-coated fiber wascommercially obtained and the inserted twist was 990 turns/meter. Thenylon muscle was two-end tethered. The operating voltage used to openand close the blinds during a 25 second cycle was a 15 V square wave (7seconds on and 18 seconds off) for the 28 cm long coiled fiber. Thelength change of the coiled fiber was a 2 cm contraction (7.1%contraction) under an applied load of 200 g, which was used to returnthe shutter to closed position. Using this contraction the shutter opensfrom closed position (15° slat inclination with respect to the verticaldirection) to 90° (the fully open position) during fiber actuation.Deployment of this thermal actuation by use of ambient temperaturechanges could deploy a pulley configuration to amplify stroke for therelevant changes in ambient temperature. If an increase in generatedforce is needed, multiple coiled fibers can be deployed that operate inparallel.

Example 23

This Example 23 demonstrated precise control of actuator position via acontrol loop. The controller used was a proportional-integral-derivativecontroller (PID controller), which is like controller types widely usedfor industrial control systems. This PID controller calculates an“error” value as the difference between a measured process variable(presently position) and a desired set point. The controller attempts tominimize the error in positioning by adjusting the process controlinputs, which is presently the applied voltage, current, or power.

Although the hysteretic behavior of shape memory actuators complicatescontrol systems, this problem did not arise for the investigated coiledpolymer fiber. A simple PID loop was implemented that accuratelycontrolled the displacement of a coiled, carbon-nanotube-sheet-wrappednylon actuator. Displacement was measured by an Omega LD701 non-contactdisplacement sensor, and input/output connected to a computer via a NIPCI-6040E acquisition card. A Labview-based PID loop controlleddisplacement to within 0.1 mm. This system was robust enough toaccurately produce user-defined sine-waves at up to 2 Hz.

Example 24

In this Example 24, the CNT wrap covering a polymer wire muscle wasshown to produce sound via the thermoacoustic effect duringelectrothermal actuation. A CNT-sheet wrapped, coiled nylon actuator(127 μm Coats and Clark D67 transparent nylon) was prepared. Instead ofapplying a DC voltage, a 5 kHz AC sine wave voltage of similarroot-mean-square (RMS) amplitude was applied to induce actuation. Due tothe high surface area and low heat capacitance of the CNT sheet, thermalactuation of the polymer wire muscle produced an audible sound at 10kHz. The output frequency was double that of the input frequency due toJoule heating at both positive and negative voltages.

Example 25

This Example 25 demonstrated the irreversible response of aone-end-tethered nylon fiber actuator to a chemical fuel. A 127 μm Coatsand Clark D67 nylon filament was wrapped with a CNT sheet containingdeposited Pt-black catalyst particles (Alfa Aesar 12755). The filamentwas then twisted until fully coiled. This process caused significantplastic deformation of the nylon, such that even when allelastically-stored twist was removed, the filament retained its coiledshape. When placed (while unrestrained at one muscle end) over a beakerof methanol, the reaction of methanol vapor and oxygen on the platinumsurface heated the actuator. This caused irreversible untwist of thecoiled structure, indicative of the temperature attained.

Example 26

This Example 26 demonstrates two-end-tethered tensile actuation for anylon fiber that was fully coiled by twist insertion and then wrappedwith helical windings of carbon MWNT sheet strips that have anoppositely directed bias angle with respect to the axis of the coil.Optical micrographs of the coiled nylon fiber before and after such dualhelical wrapping with MWNT sheet strips are shown in FIGS. 15A-15B.

A coiled nylon fiber muscles were prepared from ˜230 μm diametercommercially-available, multi-filament nylon sewing thread (Coats &Clark) by using a two-step process. First, a coiled, twist-insertednylon fiber (FIG. 15A) capable of retaining its shape when non-tetheredwas fabricated. To accomplish this, a length of the precursor sewingthread was loaded with 35 MPa stress and twisted until the thread wasfully coiled. The initial polymer fiber length was 4-5 times longer thanthe length of the coiled structure. In order to stabilize the shape ofthis coiled structure, the coiled thread was heated with a hot air heatgun to above the nylon glass transition temperature for about 5 min.Such prepared polymer fiber retained the coiled structure afteruntethering and removal of load.

In a second preparative step, the coiled fiber was wrapped with severallayers of forest-drawn MWNT sheet strip. In this procedure, the fiberwas rotated at a constant speed as a MWNT sheet was drawn from astationary forest and translated along the fiber length. MWNT sheetstrip wrapping on the coiled nylon fiber (thread) wrapping was performedtwo times in forward direction and then two times in the reversedirection to provide helically wrapped sheet ribbons that aresymmetrically plied with respect to the axis of the coiled fiber yarn.Optical microscope images of coiled fiber before and after wrapping withMWNT sheet are shown in FIGS. 15A and 15B, respectively.

For two-end-tethered actuation measurements, one end of the preparedpolymer fiber muscle was attached to a stationary metal rod, whichserved as one electrical connection, and the opposite muscle end wasconnected to a thin, flexible metal wire, which served as the secondelectrical connection provided the second yarn tether. After loading thepolymer muscle with a weight that provides 21.7 MPa, when normalizedwith respect to the cross-section of the initial non-twisted thread, thepolymer fiber muscle was actuated by Joule heating using a square-wavevoltage of variable amplitude that was applied between the stationaryrod and metal wire. The surface temperature of the muscle duringactuation was recorded as a function of time during the actuator cycleusing a thermocouple. Also, the resistance of the MWNT joule heaterduring actuator cycles was recorded using a Keithley source meter. Theactuation of the polymer fiber muscle induced by Joule heating wasdetected with a microscope equipped with a video camera. Typical timedependences of applied voltage, electrical resistance, temperature andgenerated strain for the muscle loaded with 21.7 MPa stress are shown inFIG. 16. The muscle reversibly contracted by up to 26% when heated.

Example 27

This Example 27 describes the tensile actuation of two-end-tethered,mandrel-coiled thermal polymer fiber muscles having a negative thermalexpansion in the coil direction. In order to prepare this polymer fiberartificial muscle, an 860 μm diameter nylon 6 monofilament was firsttwisted to just before the on-set of coiling under a load of 200 g. Thenit was wound around a mandrel in the same direction as the fiber twist.The coiled polymer fiber was then thermally annealed at 150° C. for 20min in order to set the structure. The polymer coil made using a 0.4 mmdiameter mandrel delivered 29% contraction under 3.5 MPa load whenheated to 140° C. (using hot air from a heat gun). By increasing thediameter of the coil, using a 2.7 mm diameter mandrel, a contraction ofup to 49% at 140° C. was achieved under a 1 MPa load.

Example 28

This Example 28 describes a thermally powered artificial muscle thatuses a coiled polymer fiber yarn having a positive thermal expansion, asa result of the method used for coiling on a mandrel. This muscleoperates to do mechanical work when expanding, rather than whencontracting. An 860 μm diameter nylon 6 monofilament fiber was firsttwisted under 200 g load to just before the on-set of coiling. Then thistwisted fiber was wound around a 2.7 mm diameter mandrel in an oppositedirection to the inserted fiber twist. This coil was positioned, under acompressive load of 50 g (which was supported by a cantilever beam), byusing a glass rod at coil center as coil guide (FIGS. 19A-19B). Heatingto provide actuation was provided by using the hot air from a heat gun.The coiled polymer muscle was two-end tethered by friction with thelower and upper surfaces. A thin thermocouple, which was positionedinside the muscle coil, was used to monitor the temperature of theactuating muscle. Upon heating to approximately 140° C. the coiledpolymer fiber muscle showed a reversible expansion, while lifting the 50g load, that exceeded 55% of the coiled muscle length.

Example 29

This Example 29 demonstrated high-rate actuation enabled by immersion ina water bath. Silver-plated nylon 6,6 (Shieldex Trading, Inc., 117/17-2ply, product number: 206121011717) was coiled and two-plied to form astable, high-stroke SZ muscle. A 100 g load of 10.8 MPa was applied andthe actuator and load were both submersed in a bath of de-ionized water.In these conditions, a resonance was found wherein the rapid passivecooling allowed electrothermal actuation at rates of 5 Hz and strokes ofaround 7%.

Example 30

This Example 30 measured the efficiency and specific power duringcontraction for a coiled polyethylene actuator. Polyethylene fiber(SpiderWire Stealth Braid 6 lb test) was wrapped with CNT sheet andtwisted until fully coiled. A 10 cm long sample weighing 8.5 mg wasused. When heated with a rapid pulse of 390 V for 10 ms to limit heatdissipation, the actuator consumed 166 mJ. In response, the actuatorlifted a 600 g weight by 0.37 mm, representing 2.18 mJ of work againstgravity (0.26 kJ/kg). This amounts to an energy efficiency of 1.32% anda specific power of 25.6 kW/kg during contraction.

Example 31

This Example 31 demonstrated actuators based on polymer fibers otherthan nylon and polyethylene. Kevlar, Nomex, polyvinylidene fluoride(PVDF) and polyester were measured via TMA (TA Instruments Q400EM) bycyclic heating and cooling between room temperature and an actuationtemperature. Kevlar and Nomex both have negative thermal expansioncoefficients. Before twisting, Kevlar contracted by 0.3% upon heating to300° C., while Nomex exhibited almost no contraction (<0.02%) whenheated to 280° C. When coiled, Kevlar and Nomex contracted by 10% and3.5% when heated to 350° C. and 280° C., respectively. In contrast, PVDFand polyester do not universally exhibit negative thermal expansionacross all temperatures. Before twisting, PVDF expanded up to 0.2% inlength when heated to 70° C., and subsequently contracted by 0.9% of itsoriginal length by 135° C. Polyester exhibited purely positive thermalexpansion, expanding by 0.4% when heated to 230° C. However, whencoiled, both materials contracted with heating, providing 10% and 16%stroke upon heating to 135° C. and 230° C., for PVDF and polyester,respectively. All coiled samples were two-end-tethered to preventrotation.

Example 32

This Example 32 demonstrated methods to helically wrap CNT sheet stripsaround fiber substrates. For samples of a finite length, the fiber wassupported between two motors. A CNT sheet drawn from a spinnable CNTforest was attached at a specific feed angle, which was varied dependingon the required wrap thickness. By rotating the fiber and translatingeither the fiber or the forest along the fiber's length, an even coatingof CNT sheet strip was applied with a constant feed angle. When lowerresistance or higher areal sheet density were required, additional wrapscould be applied at the same or different bias angles. This techniquewas also extended to allow continuous wrapping of fibers by drawing afiber substrate through a spinning apparatus which winds a CNT forestaround the fiber.

Example 33

This Example 33 demonstrated a simple design for providingtwo-dimensional displacement of a load by using an arrangement ofnon-parallel polymer fiber or hybrid nanofiber yarn muscles or musclesegments. A CNT-wrapped, coiled nylon actuator (fabricated from a 127 μmCoats and Clark D67 transparent nylon fiber precursor) was held betweentwo supports, suspending a 50 g load in the middle of the yarn. Thespacing between supports was adjusted so that the actuator formed a Vshape with a 90 degree angle, with the load supported at the vertex.Electrodes were attached at each support, and to the 50 g weight in themiddle, such that each leg of the V could be heated independently.Applying equal voltage to each leg lifted the weight vertically, whileapplying dissimilar voltages allowed the weight to move horizontally.This motion was used to move a capillary tube between acid, base andindicator containers, demonstrating automated dispensing and mixing ofsolutions.

An analogous arrangement to that of this example can be used to provideto provide three-dimension displacement of a mechanical load. Forexample, instead of using one V-shaped actuating polymer fiber (withindependently legs), two actuating polymer fibers can be deployed (againwith independently addressable legs), so as to provide independentlycontrolled displacement in the third direction. The V-shaped second yarnmuscle can optionally be identically configured as the first V-shapedyarn muscle, but oriented at 90° to the first V-shaped yarn muscle. Such2-D and 3-D muscle configurations can be used for diverse application,including, on the macroscale, microscale, or nanoscale, the displacementof probes for nano- or micro-microscopy, or moving or tilting samplesfor SEM (scanning electron microscope) microscopy or TEM (transmissionelectron microscope) microscopy. The ability to tilt can be provided byattachment of the muscles at more than one place on the item beingtilted.

Example 34

This Example 34 experimentally demonstrates thermal and electrothermalactuation of various braided or plain-weave-woven structures made fromcoiled, two-ply, SZ twisted nylon fiber muscles (which are either monoor multi-filament and are either with or without a conducting coating).

For the first described results, eight MWNT-ribbon-sheet-wrapped,coiled, two-ply, SZ nylon 6 fibers were assembled into a flat-braidtextile. The precursor fiber for the actuating fiber muscle was acommercially obtained 130 μm diameter nylon 6 monofilament (0.005 sizemonofilament from Coats and Clark). Twist was inserted in this fiber(3280 turns/m using a 30 g weight to provide fiber tension) and then thetwisted fiber was folded back on itself to make a SZ two-ply yarn. Thetwo-ply SZ yarn was helically wrapped with a forest-drawn MWNT sheetstrip, so as to provide five wrapped layers of MWNT sheet strip. Beforethis MWNT coating process the outer diameter of the two-ply SZ yarn was538 μm±2%, and after this process the diameter increased to 567 μm±3%.The typical fiber resistance of the two-ply polymer fiber afterfive-layer MWNT coating was 682 Ω/cm, and this resistance decreased to167 Ω/cm for the flat braid structure in which eight wrapped SZ nylon 6fibers operate in parallel.

Actuation was performed under constant 520 g load, using a square wave100 mA current (3 seconds on, 5 seconds off). For a 12 cm long textilesample, 1.6 cm displacement was observed, which corresponds to a 13.3%stroke. A second flat braid textile (pictured in FIG. 21A) was madeanalogously from four non-coated, coiled, two-ply, SZ nylon 6monofilaments, which could be actuated by environmentally providedheating or photothermal heating.

A round braided rope (pictured in FIG. 21C) was made by braiding eightof the above described MWNT-coated, SZ twisted nylon 6 yarn muscles. Theresistance of the entire braided structure was 83 Ω/cm. Actuation wasperformed under constant 550 g load, using a square-wave voltage of 5.5V/cm (3 seconds on and 5 seconds off). For the 7.29 cm long braid, theobserved 1 cm contraction during electrothermal heating corresponds to a13.7% stroke. Another round braid was analogously made by braiding eightSZ twisted nylon 6 yarns, but without incorporation of the MWNTelectronic conductor. This braid (pictured in FIG. 21B) could beactuated by direct thermal heating or photothermal heating.

A plain weave textile was made by incorporating ten, non-coated SZ,2-ply coiled nylon fibers (prepared as above described) in the warpdirection and cotton yarn in the weft direction. This textile can beactuated by direct thermal heating. Another plain-weave structure(pictured in FIG. 21D) was constructed by first converting acommercially available silver-coated nylon 6,6 multi-filament fiber intoa SZ, 2-ply, coiled nylon muscle. Eight such coiled muscle fibers wereincorporated in the warp direction of a plain weave textile, whilecotton yarns were in the weft direction. The textile resistance in thewarp direction was 60 Ω/cm. Actuation was performed under constant 1.3kg load using a warp direction textile length of 6.25 cm. When applyinga square wave voltage of 90 V (with 6 seconds on and 25 seconds off), adisplacement of 7.9 mm was observed, corresponding to a stroke of 12.6%.

Example 35

This Example 35 demonstrated a “breathing textile” in whichelectrothermally or thermally powered nylon fiber muscles opened andclosed porosity for a McKibben braid textile weave. The utilizedCNT-coated SZ, 2-ply coiled nylon muscles were made as described inExample 34. Eight of these muscles were braided together to make theactuator, which is located inside the McKibben braid. The weave porearea reversibly increased by up to 16% (as shown by comparing the FIG.22A and FIG. 21A photographs) when this eight-muscle-based braid waselectrothermally actuated (using a 40V square-wave voltage that was 5 son and 20 s off) under a 350 g load.

Example 36

This Example 36 investigates the effect of twist density on thermalactuation for a heat-set, SS nylon fiber in which the twist direction ofcoiling is the same as the twist direction within the fiber. An oven(150° C. for 30 minutes) was used to heat set the 300 um diameter,mandrel-coiled nylon fiber and thermal actuation was subsequentlyprovided using hot air from a heat gun. For the same applied load,thermally set SS coiled nylon yarn with a twist density below 170turns/meter (for instance, coils with a fiber twist density of 0, 100,120, and 150 turns/meter) irreversibly elongate upon heating to abovethe 150° C. heat-set temperature and a coiled nylon with a twist densityabove 170 turns/m (for example, 200 turns/m) nearly reversibly contractswhen heated and elongates when cooled. Hence, the critical twist densityis about 170 turns/m of fiber twist.

Example 37

This Example 37 conceptually demonstrated the use of a coiled nylonfiber muscle for harvesting thermal energy as mechanical energy due totemperature changes, and the use of a mechanically attached array offive cantilevered piezoelectric plates for converting this mechanicalenergy to electrical energy. This electrical energy was generated sincethe nylon fiber muscle was attached to the free ends of thepiezoelectric cantilevers. Though for convenience in the present examplethe temperature change of the coiled nylon fiber muscle was electricallyproduced, an identical arrangement can be used for harvesting thethermal energy resulting from changes in ambient temperature.

Example 38

This Example 38 demonstrates the use of a polymer-muscle-powered systemfor regulation of light or air flow based on changes of environmentaltemperature. This application embodiment uses a mandrel-coiled nylonmuscle with positive thermal expansion to do mechanical work when itexpands while under compression, which is an embodiment that can also beused for smart textiles that increase porosity when temperatureincreases. An 860 μm diameter nylon 6 monofilament fiber was firsttwisted under 200 g load to just before the on-set of coiling. Then thistwisted fiber was wound around a 2.7 mm diameter mandrel in an oppositedirection to the inserted fiber twist and heat set at a temperature thatis higher than the application temperature of 80° C. Two coils wereprepared in this way. They were positioned around two metal rods thatserved as guides for their movements. Upon heating to approximately 80°C. by using the hot air from a heat gun, the coils expanded, movingapart a set of plastic tubes that were supported by nylon filamentrunning between the turns of the coils (FIG. 23A). Upon natural cooling,the coils return to their original position (FIG. 23B), thereby closingthe shutter.

Various Features of the Invention

The present invention includes nanofiber-based yarn actuators (such asartificial muscles). The present invention further includes actuators(artificial muscles) including twist-spun nanofiber yarn ortwist-inserted polymer fibers that generate torsional and/or tensileactuation when powered electrically, photonically, chemically, byabsorption, or by other means. These artificial muscles utilizenon-coiled or coiled yarns and can be either neat or including a guest.The present invention also includes devices including these artificialmuscles.

The disclosures of all patents, patent applications, and publicationscited herein are hereby incorporated herein by reference in theirentirety, to the extent that they provide exemplary, procedural, orother details supplementary to those set forth herein. It will beunderstood that certain of the above-described structures, functions,and operations of the above-described embodiments are not necessary topractice the present invention and are included in the descriptionsimply for completeness of an exemplary embodiment or embodiments. Inaddition, it will be understood that specific structures, functions, andoperations set forth in the above-described referenced patents andpublications can be practiced in conjunction with the present invention,but they are not essential to its practice. It is therefore to beunderstood that the invention may be practiced otherwise than asspecifically described without actually departing from the spirit andscope of the present invention.

While embodiments of the invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit and teachings of the invention. Theembodiments described and the examples provided herein are exemplaryonly, and are not intended to be limiting. Many variations andmodifications of the invention disclosed herein are possible and arewithin the scope of the invention. Accordingly, the scope of protectionis not limited by the description set out above.

What is claimed is:
 1. A non-electrochemical actuator comprising: (a) afirst actuating twist-spun nanofiber yarn segment that comprisesnanofibers and an actuating yarn guest that is operable for undergoingsubstantial change in volume by a change process selected from the groupconsisting of heating, exposing to radiation, exposing to a chemical orchemical mixture, and any combination thereof, wherein said firstactuating twist-spun nanofiber yarn is either non-coiled or coiled; (b)a first attachment coupled, directly or indirectly, to the firstactuating twist-spun nanofiber yarn segment, wherein (i) thenon-electrochemical actuator is operable for providing an actuationselected from the group consisting of tensile actuation, torsionalactuation, a simultaneously useable combination of tensile and torsionalactuation, and any combination thereof; (ii) the first attachment isoperable for enabling utilization of the actuation in which theactuation is produced, at least in part, by the first actuatingtwist-spun nanofiber yarn segment.
 2. The non-electrochemical actuatorof claim 1, wherein (a) a heating provision is operably connected to thefirst actuating twist-spun nanofiber yarn segment for electricallyheating the first actuating twist-spun nanofiber yarn segment; (b) theactuating yarn guest is operable for undergoing substantial change involume when heated by the heating provision, and (c) the heatingprovision is operable for heating selected from the group consisting ofperiodic heating, non-periodic heating, and both periodic heating andnon-periodic heating.
 3. The non-electrochemical actuator of claim 1,wherein (a) the non-electrochemical actuator is a torsional actuator;and (b) the first attachment is a first torsional attachment selectedfrom the group consisting of a paddle and a torsional lever arm.
 4. Thenon-electrochemical actuator of claim 3, further comprising a secondsegment having a first end and a second end, wherein (a) the firstactuating twist-spun nanofiber yarn segment has a first end and a secondend, (b) the second end of the first actuating twist-spun nanofiber yarnsegment is connected to the first end of the second segment, (c) thefirst end of the first actuating twist-spun nanofiber yarn segment andthe second end of the second segment are torsionally tethered toprohibit torsional rotation, and (d) at least one of the torsionalattachments is made in close proximity to the mechanical connectionbetween the second end of the first actuating twist-spun nanofiber yarnsegment and the first end of the second segment.
 5. Thenon-electrochemical actuator of claim 4, wherein the second segment issubstantially non-actuating and is operable to act as a torsionalspring.
 6. The non-electrochemical actuator of claim 4, wherein thesecond segment is a second actuating twist-spun nanofiber yarn segmentthat comprises actuating yarn guest and actuates to provide torsionalrotation at the connection between the second end of the first actuatingtwist-spun nanofiber yarn segment and the first end of the secondsegment that is in the same direction as provided at this connection bythe first actuating nanofiber yarn segment.
 7. The non-electrochemicalactuator of claim 1, wherein (a) the first actuating twist-spunnanofiber yarn segment is a yarn selected from the group consisting of asingle-ply yarn, a two-ply yarn, and a four-ply yarn, (b) each of theplies in the two-ply yarn and each or the plies in the four ply-yarnhave the same direction of inserted twist, and (c) the twist directionfor plying these plies together is opposite to the twist direction ineach ply in the two-ply yarn and in the four-ply yarn.
 8. Thenon-electrochemical actuator of claim 3, wherein (a) the first actuatingtwist-spun nanofiber yarn segment is substantially filled with theactuating yarn guest, and (b) the non-electrochemical actuator furthercomprises at least one means to produce torsional actuation of the firstactuating twist-spun nanofiber yarn segment, wherein the means isselected from the group consisting of (1) wire connections that enableelectrical heating of the actuating yarn guest, (2) a radiation sourceand radiation pathway that is operable for enabling at least one ofphotothermal heating of the actuating yarn guest or the photoreaction ofthe actuating yarn guest, (3) a chemical source and chemical pathway tothe actuating yarn guest that is operable for enabling chemical exposureof the yarn guest, and (4) combinations thereof.
 9. Thenon-electrochemical actuator of claim 1, wherein the nanofibers compriseelectronically conducting nanofibers that have sufficient electricalconductivity to enable electrothermal actuation using electrical Jouleheating.
 10. The non-electrochemical actuator of claim 1, wherein thenanofibers are selected from the group consisting of carbon multi-wallednanotubes, carbon few-walled nanotubes, carbon single-walled nanotubesor few-walled nanotubes that have sufficiently large nanotube diameterthat they have collapsed into ribbons, graphene nanoribbons, derivativesthereof, and combinations thereof.
 11. The non-electrochemical actuatorof claim 1, wherein the nanofibers comprise nanofibers made by theprocess of electrostatic spinning.
 12. The non-electrochemical actuatorof claim 1, wherein the actuating yarn guest is selected from the groupconsisting of a paraffin wax, a polyethylene glycol, a long chain fattyacid, an organic rotator crystals, a silicone rubber, palladium, andcombinations thereof.
 13. The non-electrochemical actuator of claim 1,wherein the actuating yarn guest is capable of transforming duringactuation in a manner selected from the group consisting of betweendifferent solid phases, between solid and liquid states and combinationsthereof.
 14. The non-electrochemical actuator of claim 1, wherein (a)the first actuating twist-spun nanofiber yarn segment is coiled, and (b)the first twist-spun nanofiber yarn comprises actuating yarn guest oversubstantially the entire yarn length.
 15. The non-electrochemicalactuator of claim 1, wherein (a) the non-electrochemical actuator isoperable for simultaneously providing highly reversible torsional andtensile actuation, (b) the first actuating twist-spun nanofiber yarnsegment is substantially filled with the actuating yarn guest, (c) theactuating yarn guest is operable to reversibly undergo substantialchange in volume by the change process, (d) the non-electrochemicalactuator is under tensile load and able to change length and comprisesat least one of the following: (1) an actuating twist-spun coiled ornon-coiled single-ply nanofiber yarn segment or an actuating twist-spuntwo-ply nanofiber yarn segment that is tethered at a first segment endto substantially prevent translation and rotation at the first segmentend, and supports a paddle or torsional lever that is attached along theyarn segment length at a position that is distant from the tether, andcomprises a solid actuating yarn guest that is not operable tocompletely change from solid to fluid state during actuation, (2) anactuating twist-spun coiled or non-coiled single-ply nanofiber yarnsegment or an actuating twist-spun two-ply nanofiber yarn segment thatcomprises an actuating yarn guest and is tethered at the first end tosubstantially prevent rotation at the first end and attached on thesecond end to a first end of a substantially non-actuating element thatacts as torsional spring, wherein the second end of the substantiallynon-actuating element is tethered to prevent rotation of the second end,wherein one of the two end tethers prohibits end translation, andwherein a paddle or torsional lever arm is attached in proximity to theconnection between the actuating nanofiber yarn and the substantiallynon-actuating element, (3) two mechanically-connected twist-spun two-plyactuating nanofiber yarn segments that both comprise the actuating yarnguest, wherein: (i) each of the individual plies in each of the twomechanically-connected two-ply yarn segments has the same direction ofyarn twist, (ii) each of the two-ply yarn segments have opposite twistdirections for yarn twist and yarn plying, (iii) the direction of yarntwist in the first mechanically-connected two-ply yarn segment isopposite to the direction of yarn twist for the secondmechanically-connected two-ply yarn segment, (iv) the ends of two-plyyarn segments that are most distant from the connection between the twomechanically-connected two-ply yarn segments are tethered to preventtorsional rotation, (v) one of these end tethers prohibits endtranslation, and (vi) a paddle or torsional lever arm is attached at ornear the mechanical connection between the two mechanically-connectedtwo-ply yarn segments.
 16. A process comprising the steps of: (a)producing a twist-spun nanofiber yarn by inserting twist into ananofiber array selected from the group consisting of (1) a non-twistednanofiber yarn comprising oriented nanofibers; (2) a ribbon of orientednanofibers; and (3) a ribbon of oriented nanofibers converging toproduce a twist spun yarn; (b) introducing an actuating yarn guest, or aprecursor thereof, by infiltrating the actuating yarn guest, or theprecursor thereof, into the twist spun yarn; and (c) forming anactuating twist-spun nanofiber yarn comprising the actuating yarn guest,or the precursor thereof, that is operable for undergoing substantialchange in volume by a change process selected from the group consistingof heating, exposing to radiation, exposing to a chemical or chemicalmixture, and any combination thereof.
 17. A process comprising the stepsof: (a) forming a nanofiber array selected from the group consisting of(1) a ribbon of oriented nanofibers and (2) a ribbon of orientednanofibers converging into yarn; (b) introducing actuating yarn guest,or a precursor thereof, by depositing actuating yarn guest, or aprecursor thereof, on the nanofiber array; (c) inserting twist into thenanofiber array to form a nanofiber yarn that is either (1) an actuatingtwist-spun nanofiber yarn comprising actuating yarn guest, or theprecursor thereof, that is operable for undergoing substantial change involume by a change process selected from the group consisting ofheating, exposing to radiation, exposing to a chemical or chemicalmixture, and any combination thereof, or (2) a precursor twist-spunnanofiber yarn that can be converted to an actuating twist-spunnanofiber yarn comprising actuating yarn guest, or the precursorthereof, that is operable for undergoing substantial change in volume bya change process selected from the group consisting of heating, exposingto radiation, exposing to a chemical or chemical mixture, and anycombination thereof.
 18. The process of claim 16, wherein (a) theoriented nanofibers are produced directly or indirectly by a methodselected from the group consisting of (1) electrostatic spinning, (2)carbon nanotube draw from a carbon nanotube forest, (3) carbon nanotubedraw from an aerogel formed from floating-catalyst-produced carbonnanotubes, (4) solution spinning from a dispersion of nanofibers in aliquid, (5) unzipping oriented multiwall carbon nanotubes to provideoriented graphene nanoribbons, and (6) templating a material on orientedcarbon nanotubes.
 19. The process of claim 16, wherein (a) the step ofintroducing the actuating yarn guest, or the precursor thereof, isconducted on a low-twist or false-twisted nanofiber yarn having highvoid volume fraction, and (b) the process further comprises usingadditional twisting to provide a predetermined twist insertion state forthe actuating twist-spun nanofiber yarn.
 20. The process of claim 16,wherein the actuating twist-spun nanofiber yarn is a coiled, thermallyactuating twist-spun nanofiber yarn made by a method selected from thegroup consisting of (a) inserting false twist or less twist thanrequired for yarn coiling for a guest-free nanofiber yarn, (b)infiltrating a molten polymer or an uncured polymer resin into theguest-free yarn, (c) inserting twist sufficient to cause yarn coilingfor the infiltrated yarn, and (d) solidifying the polymer or curing thepolymer resin.
 21. The non-electrochemical actuator of claim 1 that isan actuating sensor comprising the non-electrochemical actuator operablefor providing an actuation selected from the group consisting of tensileactuation, torsional actuation, a simultaneously useable combination oftensile and torsional actuation, and any combination thereof, whereinthe non-electrochemical actuator comprises (a) the first actuatingtwist-spun nanofiber yarn segment that comprises nanofibers and theactuating yarn guest that is operable for undergoing substantial changein volume by a change process selected from the group consisting ofheating, exposing to radiation, exposing to a chemical or chemicalmixture, and any combination thereof; and (b) the first attachmentcoupled, directly or indirectly, to the first actuating twist-spunnanofiber yarn segment, wherein the first attachment is operable forenabling utilization of the actuation in which the actuation isproduced, at least in part, by the first actuating twist-spun nanofiberyarn segment.
 22. The actuating sensor of claim 21, wherein (a) theactuating sensor is operable for utilizing the actuation due to highlyreversible volume changes of an actuating yarn guest, and (b) theactuation enables obtaining information selected from the groupconsisting of (1) continuously varying values of temperature or chemicalexposure to be displayed or used for control purposes, (2) extremevalues of temperature to be recorded in a displayed fashion or used forcontrol purposes, or (3) the number of times a particular lower or uppertemperature limit has been exceeded to be counted in a displayed manneror used for control purposes.
 23. The actuating sensor of claim 21,wherein (a) the actuating sensor is operable for integrating exposureeffects, (b) the actuating sensor is operable for utilizing theactuation due to largely non-reversible volume changes of the actuatingyarn guest, and (c) the actuation is operable for enabling performanceof a process selected from the group consisting of integrating theeffects of temperature exposure or chemical exposure and enabling suchintegrated effect to be displayed or used for control purposes.
 24. Thenon-electrochemical actuator of claim 1, wherein the actuator isthermally-powered by a change in temperature that provides the actuationis operably caused by one of the following (i) a change in ambienttemperature (ii) a change in temperature caused by electrical heating,and (iii) heating caused by electromagnetic radiation.
 25. An articlecomprising a non-electrochemical actuator, wherein (a) thenon-electrochemical actuator is operable for providing an actuationselected from the group consisting of tensile actuation, torsionalactuation, a simultaneously useable combination of tensile and torsionalactuation, and any combination thereof (b) the non-electrochemicalactuator comprises (1) a first actuating twist-spun nanofiber yarnsegment that comprises an actuating yarn guest that is operable forundergoing substantial change in volume by a change process selectedfrom the group consisting of heating, exposing to radiation, exposing toa chemical or chemical mixture, and any combination thereof, and (2) anattachment coupled, directly or indirectly, to the first actuatingtwist-spun nanofiber yarn segment, wherein (i) that non-electrochemicalactuator is operable for providing an actuation selected from the groupconsisting of tensile actuation, torsional actuation, a simultaneouslyuseable combination of tensile and torsional actuation, and anycombination thereof; (ii) the first attachment is operable for enablingutilization of the actuation in which the actuation is produced, atleast in part, by the first actuating twist-spun nanofiber yarn segment(c) the article is selected from the group consisting of (i) athermally, photonically of chemically actuated textile or braid, (ii) athermally actuated or photonically actuated mechanical mechanism foropening and closing shutters or blinds to regulate light transmission orair flow, (iii) a thermally or photonically actuated mechanical drivefor a medical device or toy, (iv) a thermally or photonically actuatedmacro- or micro-sized pump, valve drive, or fluidic mixer, (v) athermally actuated mechanical relay for opening and closing anelectronic circuit or opening and closing a lock, (vi) a thermallyactuated torsional drive for a rotating electrode used in highlysensitive electrochemical analyte analysis, (vii) a thermally orphotonically actuated drive for an optical device, (viii) a thermally orphotonically actuated drive for an optical device that opens and closesan optical shutter, translates or rotates a lens or light diffuser,provides deformation that changes the focal length of a compliant lens,or rotates or translates pixels on a display to provide a changing imageon the display, (ix) a thermally actuated mechanical drive system thatprovides tactile information, (x) thermally actuated mechanical drivesystem that provides tactile information for a haptic device in asurgeons glove or a Braille display, (xi) a thermally or photonicallyactuated mechanical drive system for a smart surface that enables changein surface structure, (xii) a thermally actuated mechanical drive systemfor an exoskeleton, prosthetic limb, or robot, (xiii) a thermallyactuated mechanical drive system for providing realistic facialexpressions for humanoid robots, (xiv) thermally actuated smartpackaging for temperature sensitive materials that opens and closesvents or changes porosity in response to ambient temperature, (xv) athermally or photonically actuated mechanical system that opens orcloses a valve in response to ambient temperature, a temperatureresulting from photothermal heating, or a photoreaction, (xvi) athermally or photonically actuated mechanical drive that controls theorientation of solar cells with respect to the direction of the sun,(xvii) a photonically actuated micro device, (xviii) a thermally orphoto-thermally actuated energy harvester that uses changes in ambienttemperature or temperature changes produced by photothermal heating toproduce mechanical energy that is harvested as electrical energy,(xviii) a thermally actuated close-fitting garment, wherein thermalactuation is used to facilitate entry into the garment, (xix) athermally actuated device for providing adjustable compliance, whereinsaid adjustable compliance is provided by electrothermal actuation, (xx)a thermally or photonically actuated translational or rotationalpositioners, (xxi) a chemically actuated medical device, and (xxii) anactuating sensor powered thermally, by chemical absorption, or bychemical reaction that captures the energy of the sensing process tomechanically actuate.
 26. The process of claim 17, wherein (a) theoriented nanofibers are produced directly or indirectly by a methodselected from the group consisting of (1) electrostatic spinning, (2)carbon nanotube draw from a carbon nanotube forest, (3) carbon nanotubedraw from an aerogel formed from floating-catalyst-produced carbonnanotubes, (4) solution spinning from a dispersion of nanofibers in aliquid, (5) unzipping oriented multiwall carbon nanotubes to provideoriented graphene nanoribbons, and (6) templating a material on orientedcarbon nanotubes.
 27. The process of claim 17, wherein (a) the step ofintroducing the actuating yarn guest, or the precursor thereof, isconducted on a low-twist or false-twisted nanofiber yarn having highvoid volume fraction, and (b) the process further comprises usingadditional twisting to provide a predetermined twist insertion state forthe actuating twist-spun nanofiber yarn.